Transformerless dc to ac converter

ABSTRACT

A DC to AC converter is described in which DC power presented as a set of relatively floating DC power sources of differing DC voltage values is converted to a desired AC power waveform by combining the DC voltage values with sequential sets of associated ternary-valued multiplicative weights of +1, −1, or 0, and summing the weighted voltage values, so as to produce voltage points on the desired waveform at appropriate time instants.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/062,884, filed Oct. 24, 2013, which is to issue as U.S. Pat. No.9,735,703 on Aug. 15, 2017, and which is a continuation of U.S. patentapplication Ser. No. 13/103,070, filed May 8, 2011, which issued as U.S.Pat. No. 8,937,822 on Jan. 20, 2015, the disclosures of both of whichare hereby incorporated herein by reference, in their entireties.

FIELD OF INVENTION

The present invention relates generally to DC-to-AC convertors forelectric power systems, and in particular to a transformerless inverteroperative to approximate an AC output by selectively switching thepositive, negative, or zero values of a plurality of differently-valuedDC sources and summing the switch outputs.

BACKGROUND

Lower cost, high-power, efficient, DC-to-AC converters (also known as“inverters”) are of interest for solar energy economics. At the currentstate of the art as of the date of filing this CIP, DC-AC inverters havesurpassed the photovoltaic panels in cost and have become the secondhighest cost item for a solar system, next to installation labor. Forhigh efficiency and low heat dissipation, commutation of DC to produceAC preferably uses solid state switches that are either fully on orfully off, and do not dwell more than a microsecond or so in anintermediate state. Therefore it is more complicated to produce a sinewave that takes on all values between the negative peak and the positivepeak. On the other hand, producing a square wave which switches betweenthe positive peak and the negative peak produces a form of AC that isnot suitable for all loads.

Various manufacturers provide prior art DC-AC converters, which fallinto one of a few broad classes and operating modes. The class of“modified sine wave” converters maintains both the same rms and the samepeak voltage as a sine wave, while still employing only on-offcommutation. This is done by switching the voltage between the desiredpositive peak, zero and the negative peak, spending 50% of therepetition period at zero, therefore achieving both the same peak andthe same rms values as a true sine wave, and being compatible with agreater variety of loads.

Still, there are loads that do not tolerate the modified sine wave; forexample, appliances that present inductive loads, such as inductionmotors, some cellphone and laptop battery chargers, fluorescent lampsand tumble dryers, and any device with an internal power supply thatuses capacitive reactance as a lossless voltage-dropping means, canmalfunction on modified sine waveforms. Moreover, there is a potentialproblem with radio and TV interference due to the high level ofharmonics of the modified square wave converter. Such a waveform istherefore not a candidate for coupling solar-generated power into theutility network or into house wiring.

“True sine wave” is another class of prior art DC-AC inverter. Linearamplifiers provide the absolute cleanest AC power waveforms, but theirinefficiencies cause high heat dissipation in converters of severalkilowatts capacity. Moreover, linear amplifiers lose efficiency rapidlywhen operating into non-unity power factor loads. Some sine waveinverters overcome the problems with linear amplifiers by usingdigitally-synthesized waveforms, which are multi-step approximations toa smooth sine wave. One example of a step-approximation sine waveinverter is the XANTREX (formerly Trace) SW4048.

U.S. Pat. No. 5,930,128, by the current Inventor, discloses a powerwaveform generator that expresses the sinusoidal waveform as a series ofnumerical samples in a number base comprising a plurality of digits;selecting corresponding digits from each numerical sample and generatingtherefrom a waveform corresponding to the sequence of each digit; andthen using combining means to form a weighted combination of thedigit-corresponding waveforms, wherein the weights are chosen inrelation to the numerical significance of each digit. For example, usinga ternary number base, the weighting means would add the digit waveformsin the ratios 1: ⅓: 1/9 and could for example be a transformer withthese turns ratios.

U.S. Pat. No. 5,373,433 also describes using series connected,turns-ratio weighted transformer coupling of 3-level waveforms toproduce a 27-level step approximation to a sine wave. The principledescribed therein is similar to that used in the aforementioned XANTREXSW4048 inverter. The combining means disclosed in the '128 patent forcombining digit-corresponding waveforms was, in a low-frequency case, aseries connection of transformers having turns ratios in the ratios ofcorresponding numerical digits. In a high-frequency case, the combiningmeans comprised a set of quarter wave lines having characteristicimpedances in the ratios of corresponding digits.

In a device built in accordance with the '128 patent, theseries-connected transformer is the appropriate version for 60 Hz, as ¼wavelength lines are impractical at 60 Hz. However, the transformersneeded for the inventions of the '128 and '533 patents represent asignificant fraction of the total cost and weight of medium-powerconverters, and also account for a few percent loss in total efficiency.Therefore, other solutions that avoid the disadvantages and pitfalls ofthe above prior art would be useful. In particular, a solution avoidingthese low-frequency transformers would be a benefit.

Transformerless inverters are known in the prior art, particularly forutility-interactive inverters, using high-frequency switching or pulsewidth modulation to approximate a sinewave. However, a disadvantage thatarises in these conceptual converters is the imposition of thehigh-frequency switching waveform on the solar array, which cancapacitively couple through the glass cover upon touching it,potentially causing RF burn to personnel or damage to the solar panel,as well as causing the solar array to radiate substantial radiointerference. Thus a design is required that can create a more benignvoltage fluctuation on the solar array DC conductors.

Another categorization of converter relates to whether they are designedto power loads directly, or whether they are designed to feed and sellpower back into the electricity grid. A load inverter that can powerloads directly is said to operate in standalone mode, and is also calleda “standalone inverter,” while a grid-tie inverter is said to operate ingrid-interactive mode and is also called a “grid-interactive inverter.”

For safety and other reasons, the latter have to meet differentspecifications than the former, especially under fault conditions. Inparticular, a load inverter should be a constant voltage source, while agrid-tie inverter does not have a constant voltage output but must adaptto the voltage of the grid, and is a current source. Moreover, a loadinverter is always used with battery storage, and should maintainefficiency at both light and heavy loads, and have low no-load powerconsumption, so that the battery is not discharged while the inverter isidling at night. Grid-tie inverters however do not have the samerequirement for no-load power consumption, as they do not operate atnight.

A complete alternative energy installation may thus comprise a number offunctions, including load inverters, grid-tie inverters, load managementfor manually or automatically transferring load between the utility andalternative energy supplies, storage batteries, battery chargers,circuit breakers, surge protectors and other safety devices to protectequipment and wiring and eliminate the risk of electrical mishaps underconceivable fault conditions. Other than the inverters and the array,these additional components are known as “balance-of-system” components.

For high power grid-tie installations, typically 20 kw and above,3-phase inverters are preferable in order to keep the gauge and cost ofwiring down and to assist in maintaining balance between the threephases of the electricity grid. For converters over 100 Kw, 3-phase isoften mandated by the utility company. Three phase inverters using pulsewidth modulation are known from the art of solid state Motor Drives, butthey are not suitable for grid-interactive use for many reasons, andMotor Drives do not need or have ground leak detection on the DC bus,which is internal.

The total cost of balance-of-system components required in aninstallation can be significant; therefore it is an objective of thisdisclosure to describe novel designs of inverters, safety devices, andautomatic load management devices that provide a more efficient and costeffective complete installation, and which achieve cost reductions inthe electronics to complement the currently falling cost of photovoltaicpanels.

The Background section of this document is provided to place embodimentsof the present invention in technological and operational context, toassist those of skill in the art in understanding their scope andutility. Approaches described in the Background section could bepursued, but are not necessarily approaches that have been previouslyconceived or pursued. Unless explicitly identified as such, no statementherein is admitted to be prior art merely by its inclusion in theBackground section.

SUMMARY

The following presents a simplified summary of the disclosure in orderto provide a basic understanding to those of skill in the art. Thissummary is not an extensive overview of the disclosure and is notintended to identify key/critical elements of embodiments of theinvention or to delineate the scope of the invention. The sole purposeof this summary is to present some concepts disclosed herein in asimplified form as a prelude to the more detailed description that ispresented later.

New methods and apparatus for the conversion of DC power to AC power aredisclosed herein, together with smart energy management technology toprovide an efficient ‘green’ energy installation. In a first embodiment,a novel DC to AC convertor comprises a waveform generator based onexpressing the desired output voltage waveform as a series of numericalsamples in a number base, each sample being expressed as a plurality ofdigits in the number base; one or more high-frequency, bidirectional,isolating DC to DC converters to convert input DC power at a firstvoltage to a series of relatively floating DC output voltages, thefloating voltages being in the same ratio to one another as powers ofthe number base; switches connected to each of the floating DC suppliesand controlled according to a corresponding digit of a numerical sampleto generate a corresponding floating output voltage waveform, thefloating output waveforms then being directly connected in series toform the desired power output waveform having a desired repetitionfrequency thereby eliminating the need for further low frequencytransformers or other weighting means.

In a presently preferred embodiment, one of the DC voltages is chosen toequal the input source voltage so that no isolating DC-DC convertor isrequired for that supply. Preferably, it is the supply from which thegreatest average power is drawn, which is typically the highest supplyvoltage corresponding to the most significant digit of the given numberbase. The DC source, which may be a solar-array-charged battery, isconfigured to allow its positive and negative terminals to bealternately interchanged so that the positive and negative terminals arealternately connected to the grounded neutral conductor, or referencepotential terminal, which facilitates detection of ground leakage faultsin the DC circuit using an AC ground leak detector. While one of the DCinput conductors is connected to the ground, neutral or referencepotential terminal, the other DC input is routed to and processed bycircuits which produce the desired output waveform, such as a sine wave,that is output from at least one live or “hot” AC output terminal.

A second embodiment couples power produced by a solar array into theelectricity grid. In a single-phase, grid-interactive inverter accordingto the second embodiment, a switch connects the negative terminal of thesolar array to line neutral and thus to ground, or else to a referencepotential terminal, when the required output to the utility grid hot legis instantaneously positive, and a second switch, which is inhibitedfrom operating at the same time as the first switch, connects the solararray positive terminal to line neutral and thus to the ground orreference potential terminal when the required output to the utilitygrid hot leg is instantaneously negative. Third and fourth switchesalternately connect or disconnect the solar array terminal not connectedto line neutral to or from smoothing and inverter hash filters supplyingat least one AC output terminal that is connected to the utility hotleg, the pattern of connects and disconnects being determined by a highfrequency digital waveform generator such as a delta-sigma modulator,pulse-width modulator, or similar, such that the current delivered tothe utility approximates a sinusoidal current with low harmonic content.

The waveform generator timing is controlled such that the powerdelivered to the utility substantially equals the solar power availablefrom the array, and such that the delivered current is substantiallyin-phase with the utility voltage. Output relays connects the inverterto the utility only when certain conditions such as utility voltagelimits, array voltage limits, and utility frequency limits aresatisfied. When the relays are not connecting the inverter to theelectric utility, energy originating from the photovoltaic array may beused for other purposes, such as charging a battery for operating a loadinverter. The utility connection may be made via a two-pole AC GFCIbreaker such that any ground fault on either solar array terminal throwsthe breaker and isolates the array, preventing electrical mishap. Thusthe second inverter embodiment may be used alone in grid-interactivemode, or may be used together with the first inverter implementation anda rechargeable battery in order also to provide utility backup in theevent of a power outage. Such a bimodal installation may be programmedto prioritize battery charging, and when fully charged, excess solararray power is then fed to the electric utility via the grid-tieinverter.

A three-phase grid-tie inverter according to the second embodiment isalso described. The three phase inverter comprises switches to connectthe negative of the DC source to one of the set of three phase outputterminals for the whole of the period during which it is instantaneouslynegative relative to a mean voltage or to a ground, neutral, orreference potential, while the other terminals of the set of AC outputterminals are zero or relatively positive compared to the mean, ground,neutral, or reference potential terminal, alternating in a rotatingsequence with connecting the positive of the DC source to one the set ofthree phase AC output terminals for the whole of the period during whichit is instantaneously positive relative to the mean, ground, neutral, orreference potential and the other terminals of the set of AC outputterminals are zero or relatively negative to the mean, ground, neutral,or reference potential. The AC output voltages from each of the set ofAC output terminals has a unique phase, which, in the case of two ACoutput terminals may be zero and 90 degrees, or, for three AC outputterminals, may be 0, 120 and 240 degrees, while in a single phase casethe unique phase is simply 0 degrees.

A common characteristic of the inventive converters is that acommon-mode AC signal is created in-phase on both the positive andnegative DC input terminals and on all DC conductors. The AC signal isof the same frequency as the AC output in the single phase case, threetimes the AC output frequency in the three-phase case, and two times theAC output frequency in a quadriphase (0 and 90) case. A ground leakagefault in the DC circuit is thus detectable by detecting an AC leakagecurrent at this characteristic frequency, which is technically mucheasier than detecting a DC leakage current.

One embodiment relates to a DC to AC converter operative to convert DCpower to AC power having a desired voltage and waveform, and to outputthe AC power between hot and neutral output terminals. The DC to ACconverter includes a plurality of controlled switches, each having apower input connection operative to accept DC power from an associatedDC power source at an associated DC voltage. Each controlled switchfurther has a power output connection operative to output a selected oneof: (a) the associated DC voltage, (b) the associated DC voltage havingan inverted polarity, and (c) zero voltage. The switch outputs aregenerated in response to an associated ternary-valued selection signalrepresenting the multiplier values +1, −1, or 0 respectively. The poweroutput connections of the plurality of switches are connected in seriesto output a sum voltage approximating the desired AC output voltage andwaveform.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinvention are shown. However, this invention should not be construed aslimited to the embodiments set forth herein. Rather, these embodimentsare provided so that this disclosure will be thorough and complete, andwill fully convey the scope of the invention to those skilled in theart. Like numbers refer to like elements throughout.

FIG. 1 shows a top-level block diagram of a DC-AC load converter.

FIG. 2 shows a bidirectional DC-DC converter.

FIG. 3 shows a floating H-bridge for commutating a floating DC supply.

FIG. 4 shows the waveforms of four ternary digits over a repetitioncycle.

FIG. 5 shows waveforms of the 120 volt H-bridge and on the DC supply.

FIG. 6 shows a common mode hash filter.

FIG. 7 shows details of the transient response of the common mode hashfilter.

FIG. 8 shows an RFI filter used to suppress high frequencies on theoutput.

FIGS. 9A and 9B show the difference between a correct and an erroneousRFI filter.

FIG. 10 shows more detail of a load inverter design according to theinvention.

FIG. 11 shows the start-up circuit for limiting inrush current.

FIG. 12 shows the output sinusoidal waveform from an inventiveconverter.

FIG. 13 shows the output spectrum from the converter before RFIfiltering.

FIG. 14 shows the converter output noise spectrum after RFI filtering.

FIG. 15 shows the principle of a single-phase grid intertie inverter.

FIG. 16 shows the principle of a 3-phase grid intertie inverter.

FIG. 17 shows a complete solar installation for grid-interactive useonly.

FIG. 18 shows a complete solar installation using a standalone inverter.

FIG. 19 shows a bimodal solar electric installation.

FIG. 20 Equivalent circuit of a photovoltaic (solar) cell.

FIG. 20A shows the I-V characteristics of a solar panel.

FIG. 20B shows temperature dependence of solar panel characteristics.

FIGS. 21A, B and C show the DC-DC converter waveforms for reducingstandby current.

FIG. 22 shows the outline schematic of a smart load center.

FIG. 23 shows three phase waveforms.

FIG. 24 shows another arrangement of a 3-phase grid intertie inverter.

FIG. 25 shows a modified doublet and core flux waveforms.

FIG. 26 shows a vector diagram of a generator back-feeding the grid.

FIG. 26A shows the vector diagram for rectifier mode.

FIG. 27 shows a remote-controlled solar combiner and DC disconnect.

DETAILED DESCRIPTION

For simplicity and illustrative purposes, the present invention isdescribed by referring mainly to an exemplary embodiment thereof. In thefollowing description, numerous specific details are set forth in orderto provide a thorough understanding of the present invention. However,it will be readily apparent to one of ordinary skill in the art that thepresent invention may be practiced without limitation to these specificdetails. In this description, well known methods and structures have notbeen described in detail so as not to unnecessarily obscure the presentinvention.

FIG. 1 depicts a DC-to-AC conversion apparatus according to a firstembodiment, comprising an input (100) for a floating DC power source,for example 120 volts DC from ten 12-volt rechargeable batteriesconnected in series; a pair of output terminals (150) for the AC output,one of which may be connected to the grounded conductor or neutral ofthe AC load; a bidirectional DC-DC converter (110) for converting the DCinput to a number of floating DC output voltages, and a set of reversingswitches 120 a-120 d controlled by opto-isolated driver circuit 200.While one of a pair of single phase power conductors is normally agrounded neutral conductor, the inventive converter may alternativelyprovide a floating output relative to an arbitrary reference potentialterminal. The bidirectional property of the DC-DC converter 110 impliesthat power may instantaneously flow in either direction at any pair ofinput or output terminals. If the current flows out of a positiveterminal, the direction of power flow is “out,” while if current flowsinto a positive terminal, the direction of power flow is “in.” The DC-DCconverter is of a substantially lossless, switching type, implying thatall power that flows in must come out, although the converter mayoptionally contain energy storage capacitors such there may be aninstantaneous imbalance between input and output power as the capacitorsare accumulating or releasing energy.

In this example, it is assumed that the inverter output waveform to begenerated is represented as a sequence of numerical samples, eachnumerical sample value being expressed with four digits in the ternarynumber system, e.g. (T4,T3,T2,T1), wherein each digit Ti (i=1 . . . 4)can only take on one of the three values: −1, 0 or +1. In correspondencewith the place significance of the different ternary digits, a number offloating DC supplies are generated with ratios 1:⅓: 1/9: 1/27. Assuminga 120-volt DC input source, the floating DC supplies generated byconverter 100 are therefore 40 v, 13.33 v and 4.44 volts, which arerespectively ⅓rd, 1/9th and 1/27th of the nominally 120 volt floating DCpower source. The sum of the DC input and all outputs of the DC-DCconverter is 120+40+13.33+4.44=177.77 volts. This is the peak voltagethat could be generated at the AC output (150), and corresponds to auseful sine wave output voltage of 125.7 volts rms. If necessary, allthe voltages can be scaled to produce other output voltages, for example100v, 115, 120 v, 125 v, 220 v, etc., while still maintaining thepower-of-3 ratios between the floating supply voltages. Other voltageoutputs or waveforms (within the maximum available peak voltage of allDC supplies added together) may alternatively be generated by choosingthe appropriate sequence of ternary digits. For example, the inventioncould be used to produce an output waveform for driving a vibrationtable for mechanical testing purposes, the waveform being eithernon-repetitive, or having a desired repetition frequency.

Other number systems than ternary could be used; for example, the binarynumber system could be used, or the quaternary number system could beused in which digits take on the values −3, −1, +1 or +3. However,ternary is of slightly lower component complexity per waveform step andis therefore the presently preferred choice.

Continuing to refer to FIG. 1, the floating DC input and the floatingoutputs from DC-DC converter (110) each feed respective polarityreversing switches 120 a, 120 b, 120 c and 120 d. The outputs of theswitches are directly connected in series to the AC output 150, so that,by either inverting the selected polarity of each DC supply or not, ornot selecting it, in accordance with a respective one of the ternarydigits T4, T3, T2, T1, the series-connected output can be any of the 81values (120T4+40T3+13.33T2+4.44T1) volts. For example, if all switchesselect the positive polarity, the output voltage will be120+40+13.33+4.44=177.77 volts. If however the 4.44 volt switch iscontrolled to feed straight through, the output voltage will be120+40+13.33=173.33 volts. If the 4.44 v switch is controlled to reversethe polarity, the output voltage will be 120+40+13.33-4.44=168.88 volts.By appropriate control of the switches therefore, any output voltagebetween −177.77 and +177.77 volts, in steps of 4.44 volts, can beproduced. It is important to note that, when a DC source polarity isselected to oppose the output voltage and therefore the current flow,power is feeding backwards into the DC-DC converter, which musttherefore be of a bi-directional design using for example synchronousrectifiers.

It is a significant advantage to arrange that the voltage correspondingto the most significant ternary digit, or from which the greatest poweris drawn, comes directly from the DC input, and does not pass throughthe DC-DC converter, as the DC-DC converter then only has to convert theremaining fraction of the total power. Most of the AC output power thencomes directly from the DC input source, which improves the totalconversion efficiency.

In order to generate a 60 Hz step-approximation to a 125.7 volt rmssinewave, the switches are controlled to select sequentially among the3⁴=81 possible voltage levels from −77.77 to +177.77 volts and backagain, repetitively in the proper sequence and at the proper times. Onecycle therefore comprises nominally 2×81 voltage steps, so that thenumber of level changes per second is approximately 2×81×60=9720. Thisis somewhat rapid for mechanical switching means such as relays, rotarycommutators, or cam-actuated contacts, but is well within the capabilityof semiconductor switches which can operate 100 times faster than therequired speed.

FIG. 3 depicts a full H-bridge of N-type power MOSFETs, which is thepresently preferred configuration for switches 120 a-120 d. Four N-typeMOSFET power transistors labeled Tr(a), Tr(b) Tr(c), Tr(d) are connectedbetween the DC input +ve terminal and the −ve terminal. By turning ontransistors Tr(a) and Tr(d) while turning off transistors Tr(b) andTr(c), the DC +ve input is connected to output pin 163 while DC −veinput is connected to output pin 164. Conversely, by turning ontransistors Tr(b) and Tr(c) while keeping transistors Tr(a) and Tr(d)off, the DC +ve input is connected to output terminal 164 while the DC−ve input is connected to output pin 163. A pass-through state iscreated either by turning on transistors Tr(a) and Tr(c) while holdingTr(b) and Tr(d) off, which connects the DC +ve input to both outputterminals 163 and 164, or by turning on transistors Tr(b) and Tr(d)while holding Tr(a) and Tr(c) off, which connects the DC −ve input toboth output terminals 163 and 164. These two pass-through states bothgive zero voltage output between terminals 163 and 164, but differ as towhich polarity of the floating DC input is connected to the outputterminals. Both pass-through states are used in the invention atdifferent times.

A MOSFET is turned on or off by applying a positive or (negative orzero) voltage between its gate and its source terminals. Since thesources of the upper MOSFETs Tr(a) and Tr(c) of the H-bridge areconnected to different ones of the H-bridge outputs, and the sources ofthe lower two MOSFETs Tr(b) and Tr(d) of the H-bridge are connected tothe negative of the DC supply, three different relatively floatingsupplies are required for the gate drivers IC(a), IC(b), IC(c) andIC(d). IC(b) and IC(d) may use the same gate driver supply, but IC(a)and IC(c) create separate supplies using bootstrap diodes D1 and D2 tocharge capacitors C1 and C2, respectively. Further discussion ofbootstrapping may be found in FIG. 4 of Intersil Application NoteAN9324.4 dated March 2003, by George E. Danz, the disclosure of which isincorporated herein by reference in its entirety.

Due to the reference potentials for the gate drivers being different,even within the same H-bridge, all gate drivers are preferablyopto-isolated. A suitable opto-isolated gate driver is Fairchild partnumber FOD3180. An opto-isolator comprises a light-emitting diodeilluminating a phototransistor. When current is passed through the lightemitting diode at its input, the output will be enabled. Since it isimportant that TR(a) and TR(b) should never be turned on at the sametime, as this would short the DC input, this is rendered impossible byconnecting their gate driver input LEDs back-to-back. Likewise, the gatedriver input LEDs for TR(c) and TR(d) are connected back-to-back. Acontrol input 161 is conditioned by two inverting AND gates to produceopposite polarity output signals which connect through resistors R3 andR4 to the LEDs of gate drivers IC(c) and IC(d). A logic 0 at pin 161will enable IC(d) to turn on TR(d) and switch TR(c) off. A logic 1 atpin 161 will enable IC(c) to turn on TR(c) and switch TR(d) off. A logic0 at input pin 162 will produce logic 1's at both NAND gate outputs andthus no current will flow in either LED, turning both TR(c) and TR(d)off. The latter state is useful for fast shutdown upon fault events.

If the H-bridges of FIG. 1 are controlled according to a fixed timing togenerate step approximations to a sine wave, the output voltage willsimply be proportional to the DC input voltage, e.g., 125.7 volts rmsoutput for 120 volts DC input, or 104.75 volts rms output for 100 voltsDC input, or 146.65 volts rms output for 140 volts DC input. The range100 volts DC to 140 volts DC represents the range of voltages that couldbe produced by 60 lead-acid cells in series between the two extremestates of (a) substantially fully discharged and (b) fully charged andreceiving a bulk or absorption charge. It may not be desirable to allowthe AC output to vary so much over this range of DC input voltages.Accordingly, a method is used to maintain more constant AC outputvoltage in the face of varying DC input.

Instead of controlling the switches in a fixed timing sequence, acontroller determines the time at which a sine wave output of desiredamplitude passes from being nearer one voltage step to being nearer anadjacent voltage step, given the actual DC supply voltages, and maycontrol the switches to change from the previous voltage step to theadjacent voltage step at the determined time. A simple implementation ofthe foregoing comprises pre-computing a sequence of switching times foreach of a number of possible sub-ranges of the DC input voltage; forexample, a first series of switching times for the range 100-110 volts;a second series for 110 to 120 volts; a third series for 120-130 volts,and a fourth series for 130 to 140 volts. Moreover, the series ofswitching times can be computed to give a nominal output voltage of 120volts rms instead of 125.7 volts.

This method of determining a series of switching times to produce aconstant 120 volt AC sine wave output with various DC input voltages maybe extended to operate over the DC input voltage range 100 to 327 volts.At 327 volts, the 170-volt peak value of a 120 v rms AC sine wave isproduced with a 4-digit ternary number of 1,−1,−1,−1, which illustratesthat the highest voltage H-bridge still makes its maximum voltagecontribution of 327 volts, but with the other three H-bridgesbacking-off this voltage by −109 volts, −36.3 and −12.1 voltsrespectively. Under these circumstances the highest voltage H-bridge isonly contributing current for short periods at the very peaks, so themajority of power flow passes through the DC-DC converter, reducingefficiency and maximum output rating. For such a converter, thespecification should indicate an efficiency and a maximum power deratingfactor as a function of input voltage. Input voltage ranges up toperhaps 200 volts may still be accommodated however with most of theoutput power coming directly from the DC input and only a fraction beingsupplied by the DC-DC converter, and therefore without significant lossof efficiency or power derating. Utilizing the extended input voltagecapability is one way to construct a bimodal system using the same arrayvoltage for both the grid-tie and the standalone modes, as will bedescribed more fully herein.

The above example of an extreme DC input voltage of 327 V suggestsanother embodiment of the inverter. When the highest H-bridge voltage is327 V, the second highest is 109 V. This suggests an inverter whereinthe input voltage was the second highest floating DC source, and thebidirectional DC converter generated floating outputs of 3 times, ⅓rdand 1/9th of the input. Thus in one embodiment a converter for a nominal96-volt battery powers the second highest voltage H-bridge directly, andthe DC-DC converter generates 3×96=288 V, 96/3=32 V, and 32/3=10.67 Vfor the other H-bridges. Each inverter design is thus optimized for anintended range of DC input voltages. When the DC input voltage isoutside the design range, a fault detector may apply a logic zero to pin162 of FIG. 3 and to a corresponding pin on the other half of theH-bridge and to all H-bridges, thus instantly halting operation of theconverter until the input voltage once more lies consistently within thedesign range.

FIG. 12 shows the output sinusoidal waveform from the converter producedby controlling the switches in the proper sequence, when the DC input is126 volts (a fully charged, 60-cell lead acid battery) and the desiredAC output is 120 volts rms. For 60 amps AC output (7.2 kilowatts ofoutput power), DC-to-DC converter (11) supplies only 1274 watts of thetotal power, the remainder coming directly from the 126 volt DC inputvia only H-bridge 120 a, which switches at only 60 Hz. Switching lossesare therefore negligible for the bulk of the power flow.

The use of floating DC supplies of 120, 40, 13.33, and 4.44 volts to theinputs of the H-bridges, plus the use of opto-isolators to isolate thepower MOSFET control lines, means that the outputs provided from theH-bridges are also floating, and can therefore be joined in seriesdirectly—that is, without further coupling means or weighting means, toprovide an output voltage of 120×T4+40×T3+13.33×T2+4.44×T1 where (T4,T3, T2, T1) is the four ternary digit representation of the desiredinstantaneous output voltage from the converter. The ability to join theH-bridge outputs in series directly confers the benefit that theinventive DC-AC converter scheme requires no 60 Hz transformers, whichreduces weight in particular, but the main benefits are reduced cost andsize, and improved efficiency.

If the DC input voltage is not 120 volts, but in general is denoted byVdc, then the formula for the instantaneous output voltage isVdc(T4+T3/3+T2/9+T1/27).

For a greater number of digits than 4, for example n digits T1 . . .T(n), the formula for instantaneous output voltage is

T(n)+T(n−1)/3+T(n−2)/9++T1/3^((n-1)) times the DC input voltage if thedirect current source voltage is the greatest of the floating DCsupplies or alternatively3T(n)+T(n−1)+T(n−1)/3++T1/3^((n-2)) times the DC input voltage if thedirect current source voltage is the second greatest of the floating DCsupplies, or even more generally3^(m)[T(n)+T(n−1)/3+T(n−2)/9++T1/3^((n-1))] where the power m is zero ifthe highest voltage is directly equal to the DC input voltage; m=1 ifthe DC input voltage is the second highest of the floating supplyvoltages; m=2 if the DC supply voltage is the third highest if thefloating supply voltages, and so forth.

FIG. 4 illustrates the behavior of the four ternary digits T4, T3, T2,T1 when generating a 120-volt rms AC output when Vdc=126 volts, as wasused to produce the output waveform of FIG. 12. For this particularinput and output voltage, it may be seen that the desired peak voltageof 120√2=169.7 volts is approximated with T4=T3=1 and T2=R1=0, giving126(1+1/3)=168 volts. This is nearer 169.7 V than the next higher stepof 172.44 V with T4=T3=T1=1 and T2=0. Thus small changes in the DC inputvoltage may require substantial changes in the waveform to maintain anabsolutely constant 120 volt rms output; however such accuracy is notrequired and a finite number of precomputed waveforms may be used aspreviously described.

Some hysteresis in selecting the best waveform can be provided so as toprevent the converter nurdling, which describes the sound of periodicchanges in inverter hum when inverters alternate between differentwaveforms. In any case, a selected waveform is used for at least anentire cycle to ensure symmetry between the positive and negativehalf-cycles and to ensure zero net DC bias on the output.

FIG. 4 also illustrates that the most significant DC supply voltage isnot used (i.e., T4=0) around the zero crossings of the sine wave. ThusH-bridge 120 a is programmed to the “pass-through” state when T4=0. Inthe pass-through state, either the DC input positive line or the DCinput negative line is connected to both outputs of H-bridge 120 a. Itis deliberately chosen that one H-bridge 120 a output provides theneutral or grounded conductor output of the converter while the hot legis provided by the end of the series H-bridge string, i.e., from anoutput of H-bridge 120 d. This results in either the DC input positiveline or the negative line being connected to neutral (the groundedconductor) in the pass-through state.

By choosing to form the pass through state by connecting the positive DCinput to neutral in the pass-through period following a positive halfcycle where T4 was previously equal to 1, and choosing to form thepass-through state by connecting the negative of the DC supply to theneutral/ground after a negative half cycle when T4 was equal to −1, thewaveform on each DC input terminal is a symmetrical square wave rangingfrom 0 to +120 V on the positive terminal alternating with 0 to −120 Von the negative terminal. Each DC input terminal may therefore bedescribed as having a voltage of 60 volts DC mean with a 60 Hz, 60 voltpeak square wave superimposed. When connected to a solar array, such aconfiguration is in conformance with the NEC electrical code (2008)provided a warning label is attached indicating that both terminals ofthe array and DC circuit are ungrounded and thus hot. An appropriatecolor code for wiring to the array is black for negative and red forpositive, as both colors are standard colors for AC hot legs.

When the above-described pass-through states are used, the resultingcommon-mode square wave on the DC inputs will be slightly phase-shiftedrelative to the AC output. If the square wave shall be in phase with theAC output, then the pass-through state of H-bridge 120 a can be createdby connecting the positive DC input terminal to neutral/ground for thefirst half of the pass-through state after T4=1 and connecting thenegative of the DC input to neutral/ground for the second half, andvice-versa for the pass-through state following T4=−1. In this way, thesquare wave positive-to-negative transition coincides with the zerocrossings of the AC output, as shown in FIG. 5.

At time T1 on the left, the AC output is negative and the mostsignificant ternary digit equals −1, and the DC supply +ve line isconnected to neutral via MOSFET Tr(a). Accordingly the TR(a) drivewaveform is at a logic 1 level, corresponding to MOSFET Tr(a) being ON.When TR(a) is ON, TR(b) must be off to avoid shorting the DC supply,which is guaranteed by connecting their opto-isolator LED'sback-to-back. As the AC output rises towards zero, the most significantternary digit goes to zero at time T2. At that point TR(c) is turned onand Tr(d) is turned off so that a pass-through state is created whileleaving the DC positive line connected to neutral. The AC output goesthrough zero at the center of the period during which the ternary digitis zero at time T3. To synchronize the common mode AC component on theDC supply lines with the AC output, the negative of the DC input shallnow be connected to neutral instead of the positive, while maintaining apass-through state. Thus Tr(b) turns on, and Tr(a) off, whilesimultaneously Tr(c) turns back on and Tr(d) turns off. This results inthe negative of the DC input being connected to neutral and also theH-bridge output is connected to the DC supply so that there is no netvoltage supplied to the output, in conformity with the most significantdigit still being zero.

At time T4, the most significant ternary digit becomes +1. At that pointthe negative of the DC input is left connected to neutral while theH-bridge output shall be connected to positive. Thus Tr(c) and Tr(d)change state. The situation now remains constant until the ternary digitnext goes to zero at time T5, whereupon Tr(c) and Tr(d) change stateagain to create a pass-through state. At time T6 in the middle of theperiod during which the ternary digit is zero, Tr(a) and Tr(b) changestate requiring Tr(c) and Tr(d) also to change state to maintain thepass-through state until time T7. At time T7, Tr(c) and Tr(d) changestate to deliver a negative voltage to the H-bridge output when the mostsignificant ternary digit becomes equal to −1. The just-describedsequence then repeats indefinitely from time T8.

If an inverter is designed in which the supply voltage (100) is not thegreatest of the floating supply voltages, then the common-mode waveformcaused on the DC inputs will no longer be a simple square wavecorresponding to the most significant ternary digit, but will have awaveform determined by the lesser significant ternary digit associatedwith the lesser supply voltage. Likewise, if the invention is used toproduce other waveforms for other applications, then a more complexcommon-mode waveform related to the sequence of a particular ternarydigit is induced on the DC conductors. When such a waveform is used asthe probe signal for ground leak detection on the DC conductors, it maythen be beneficial to detect leakage current using a correlator thatcorrelates for that particular waveform.

To obtain a sine wave output voltage, controller 200 of FIG. 1represents the sine wave as numerical samples expressed in the ternarynumber system. When using a finite number of digits, it might benecessary to approximate the desired output voltage by the nearestternary combination, and there are many ways in which an approximationcould be constructed. For example, a ternary digit can be jitteredbetween two adjacent values on successive instants of a high-frequencyclock in order to approximate an intermediate value. This method ishowever reserved for a variation in the inverter design in which one ofthe ternary digits, e.g., the least significant digit T1, could beomitted, along with the associated components and the 4.44 volt floatingsupply, the value of T2 then being jittered between adjacent values at ahigh frequency so as to create a mean voltage between the 13.33 voltsteps, thereby reducing component count. A low-pass LC interpolationfilter is then connected at H-bridge 120 c output to smooth thewaveform. Frequent switching can be a source of additional switchinglosses, however switching only a low voltage supply such as 13.33 voltsproduces much less switching loss than jittering the 120-volt supply.

A second approach is to choose the nearest ternary approximation to thedesired instantaneous output voltage at a sequence of successive,equi-spaced time intervals generated by a sampling clock. Yet anotherapproach, which is the presently preferred approach, is to keep theternary value fixed as long as it is the nearest approximation to theexact voltage, and to increment it or decrement it to the next adjacentvalue only at the instant that a different value becomes the nearestapproximation. Storing the approximately 81 time-values of this sequenceof switching instants requires much less memory than storing the ternarydigits at all 32768 clock instants.

One method of producing the ternary digit sequence is as follows: Letcontroller 200 comprise a crystal reference oscillator running at2¹⁵×60=1,966,080 Hz. The oscillator clocks a 15-bit divider to produce a15-bit address to a 32768×16-bit Read Only Memory (ROM) containing theprecomputed waveforms required on the FET control lines of the H-bridgesduring one 60 Hz cycle. Symmetries, such as +/− symmetry, could beexploited to reduce memory size, if important. The buffered memoryoutput bits drive the opto-isolators that drive the FET gates of eachH-bridge to generate one of its three output voltage states at eachinstant and in the correct sequence.

The presently preferred approach uses a time counter for counting of theorder of 32768 steps over a 60 Hz cycle, an address counter, and memoryto store a count and an associated set of MOSFET drive signals. Theaddress counter is initialized to the first address at an appropriatepoint in the power-up procedure and a set of MOSFET drive signals areread from the memory at that address, together with the next count. Whenthe time counter reaches the next count, the address is incremented andthe next set of MOSFET drive signals is read from memory, together withthe next count value. In this way, the number of sets of signal valuesstored is only of the order of the number of waveform steps. Eithermethod can be implemented in a suitable microcontroller, such as an80C51, which contains suitable time-counters.

Using the above methods, it is relatively easy to construct a converterthat can be selected to generate 50 Hz or 60 Hz, or even 400 Hz power byvarying the oscillator frequency, the memory contents, or both, orstoring waveforms over a lowest common multiple period of the desiredfrequency selections, or computing the ternary digit sequences in realtime.

The ternary representation of a sine wave of peak voltage Vo volts whenthe DC input voltage is Vdc may be computed as follows:

-   -   Divide up one cycle of 360 degrees into 32768 equal increments        0, θ, 2θ, 3θ . . . 32767θ, where θ=360/32768 degrees.    -   Compute the value A equal to Vo times the SINE of each angle.        Then, for each angle:        -   if A is greater than or equal to +Vdc/2, then T4=1;        -   if A is less than or equal to −Vdc/2, then T4=−1;        -   else T4=0.    -   Subtract Vdc times the just-determined value of T4 from A to        obtain the remainder in A.        -   If the remainder in A is greater than or equal to Vdc/6            volts, then T3=1;        -   if A is less than or equal to −Vdc/6 volts, then T3=−1;        -   else T3=0.    -   Subtract Vdc/9 times the just determined value of T2 from A to        get the final remainder in A.        -   If the final remainder A is greater than or equal to Vdc/54            volts, then T1=1;        -   if A is less than or equal to −Vdc/54 volts, then T1=−1;        -   else T1=0.    -   Repeat the above determination of ternary words (T4,T3,T2,T1)        for each of the 32768 angular increments.

The above algorithm can also be executed using different voltagethresholds, corresponding to other DC input voltage levels, in order todetermine a ternary digit sequence that will yield the desired AC rmsoutput voltage for the other DC input voltages.

Having determined the ternary values for each clock period, the H-bridgedrive signals required for that ternary value are then stored along withthe clock count at which they are invoked, thus occupying a much smallermemory. With four H-bridges, the drive signals are conveniently packedinto one byte, and slightly more than 81 entries result due to usingboth pass-through states for the most significant H-bridge around zerocrossings, as shown in FIG. 5, which correspond to the same ternarydigit of zero. In effect, two different forms of zero are used for themost significant ternary digit. In some cases it is possible thatadditional memory entries could be created in the event that smalltiming differences were needed between switching different H-bridges inorder to minimize glitches.

The above ternary approximation algorithm can be executed for any set ofgraduated supply voltages, and for any desired output waveform orvoltage. The peak output voltage should however be less than the sum ofthe floating DC supply voltages, if clipping of the waveform is to beavoided.

A number of ternary representation sequences can be precomputed andstored for different ratios of sine wave output voltage to DC supplyvoltage, in steps of a few percent for example, the selection amongwhich then provides the means to regulate the output sine wave voltageagainst changes in DC supply voltage.

The algorithm may alternatively be executed in real time using amicroprocessor or digital signal processor. For self-test purposes, theconverter is equipped with an A-to-D converter that measures and checkseach voltage step against limits during the start-up sequence, and mayalso periodically recheck each voltage step during normal operation, forexample, by sampling and digitizing a voltage step level just before astep to the next voltage level in sequence is taken. Then the time atwhich the just-measured voltage step is the best approximation to thedesired instantaneous sine wave level can be computed, and if differentthan the currently stored value, the stored value may be updated. Bystepping to a higher voltage step a little earlier, or a lower voltagestep a little later, a small increase in the output rms voltage isachieved, while stepping to a lower voltage step earlier or to a highervoltage step later, a small decrease in rms output voltage is obtained.In this way, a continuous and fine regulation of the output rms voltagemay be obtained.

It is also possible to use a clock frequency that is different than apower of 2 times 60 Hz. There may then be more or fewer than 32768 clockcycles in one converter cycle. In a preferred implementation, an 80C51microprocessor clock is 11.0592 MHz, giving an internal clock frequencyof 11.0592/6 MHz, which is 30720 clocks in one 60 Hz cycle. Consequentlythe ternary digit sequence is calculated in steps of 360 degrees dividedby 30720. The converter may also be provided with other stored sequencesthat can be selected to generate 50 Hz or 400 Hz outputs, which haverepetition periods of 36864 and 4608 counts respectively, when using acrystal frequency of 11.0592 MHz.

FIGS. 10 and 11 give more circuit detail of the load converter. FIG. 11gives detail of the DC input circuit, showing the entry of the DC +veand −ve lines through a common mode filter 200 into a start-up circuitcomprising start-up relay RLY1 (203), 100-watt lamp B1 (204),photo-sensor (206) and main power relay RLY2 (205). DC power flowsthrough main power relay RLY2 (205) when closed and through 100A currentmeasuring shunt (206) to 60 KHz bidirectional DC-DC inverter (110) andto 120-volt H-bridge 120 a. Bidirectional DC-DC inverter 110 generatesthe floating DC voltages of 40 v, 13.33 v and 4.44 volts for the otherthree H-bridges 120 b, 120 c, and 120 d.

Each of the DC voltages produced by the bidirectional DC-DC inverter isdecoupled with a substantial reservoir capacitor, which however is notessential to the operation of the inverter. There are two advantages ofusing reservoir capacitors: first, to reduce copper losses in the 60 KHzinverter transformer windings by smoothing out current demand, andsecond, to reduce low-load or no-load standby power. Both of these willbe explained in further detail herein. Upon switch on, these reservoircapacitors would cause a substantial and potentially damaging in-rushcurrent to flow for some fraction of a second; the purpose of thestart-up circuit of FIG. 11 is to reduce such in-rush current to safevalues. The start-up circuit also permits the whole inverter to bepowered up stage-by-stage, while checking for faults at each stage, sothat start-up may be aborted if faults exist that could cause a cascadeof other component failures.

Referring to FIG. 11, the DC + and − input terminals are connectedthrough common mode filter 200 to control circuit and auxiliary powersupply unit 202 via a 250 mA fuse 201. Upon applying a DC input to theinput terminals, auxiliary PSU 202 starts operating and powers up thecontrol circuit, which may contain a microprocessor. After performingself checks to verify that the control circuit initialized itselfcorrectly, ensuring that all output relays and the start-up and mainpower relay are in the OFF position, and all H-bridge transistors areoff, the control circuit closes start-up relay RLY1 (203), the coil ofwhich is powered from the already operating auxiliary PSU (202), toconnect the DC positive line through 100-watt lamp (204) to the input ofthe 60 KHz bidirectional DC-DC converter (110). The DC-DC converterpower transistors and synchronous rectifier transistors are commutatedat this point so that DC flowing through lamp 204 will be converted tothe desired output voltages, charging all reservoir capacitors at a ratedetermined by the limited lamp current. For the purpose of analysis, thetotal reservoir capacitance transformed through the 60 KHz invertertransformer ratio may be lumped into a single value, shown dotted inFIG. 11. The current required to charge this capacitor is sufficient tolight the 100-watt lamp for a second or so, the lamp dimming as thecapacitor charges towards 120 V. The lamp 204 thus limits the in-rushcurrent to perhaps 7 amps when the filament is cold, falling to 0.8 ampswhen the filament is at full brightness, and then falling further as thecapacitance charges up towards 120 V. The charging is faster thanexponential, as the resistance of the lamp reduces from 150 ohms at fullbrightness to about 18 ohms as it cools. The use of a lamp instead of aseries resistor has this benefit of faster charging, and also is able todissipate 100 watts as long as necessary, whereas a 100 watt resistorwould need heat-sinking and would be more expensive.

If any fault exists which causes the 60 KHz inverter, or any H-bridge,which is supposed to be off, to take current, then the no-load currentthrough the lamp will not drop to zero, and will be high enough to causethe lamp to remain partially lit. After allowing 2 seconds for normalstart up, such a fault condition is detected using phototransistor 206,which is in close proximity to lamp 204, and the control circuit abortsstart up and opens start-up relay 203 upon detecting the abnormalcondition. Assuming, however, that the lamp does not remain lit, aftertwo seconds the control circuit concludes that all reservoir capacitorshave been successfully charged and that no abnormal no-load currentexists. The control circuit also requires to have seen the lamp lightbriefly after the start-up relay is closed, thus verifying the integrityof the lamp and the phototransistor. If the lamp does not light brieflyafter closing the start-up relay, the start-up sequence may be aborted.Automatic retries may optionally be programmed to occur up to a limit.

After the reservoir capacitors are charged, the main inverter H-bridgesare operated off load with the AC output relays open. FIG. 10 shows moredetail of the complete single-phase load converter. Referring to FIG.10, the DC input enters via common-mode filter 200 to the startupcircuit just described with the aid of FIG. 11. The start-up circuitswitches the filtered DC input through a 100-amp current shunt (101-5)to 60 KHz inverter 110-1 and 120 V H-bridge 120 a. 60 KHz inverter 110-1together with synchronous rectifiers 110-2, 110-2, and 110-4 constitutethe bidirectional DC-DC converter of FIG. 2. Each synchronous rectifierhas an output reservoir capacitor and is connected to an associatedH-bridge, 120 b, 120 c, or 120 d. The four H-bridges are driven by fourcorresponding sets of four opto-isolated MOSFET drivers (222). Eachsynchronous rectifier uses two further MOSFET drives and the 60 KHzinverter uses two MOSFET drivers, making 26 in total. The MOSFET driversrequire 12-15 volt power supplies. Two MOSFET drivers can use the same15-volt supply if they drive MOSFETs that have their sources connected.Thus, using the transistor identifications of FIG. 3, one 15-volt supplyis required for H-bridge 120 a TR(b) and TR(d); another is required forH-bridge 120 b TR(b) and Tr(d) and the transistors of its associatedsynchronous rectifier 110-2; a third is required for H-bridge 120 c andits associated synchronous rectifier 110-3; a fourth for H-bridge 120 dand rectifier 110-4 and a fifth for 60 KHz inverter 110-1. Otherisolated supplies may be required for microprocessor controller andwaveform generator 202-2 and for various overcurrent, undervoltage, andovervoltage sensors. All these supplies are produced by auxiliary powersupply 202-1. Auxiliary supply 202-1 comprises a low-power, switch-moderegulator producing high frequency AC which is distributed to thelocation on the printed circuit board at which an isolated supply isneeded, where it is transformed to the desired voltage using a smalltoroidal isolating transformer followed by a diode rectifier.Distributing AC and isolating and rectifying it at destination has thebenefit of reducing PCB tracking congestion.

The output from H-bridge 120 d is coupled back to controller 202-2 wherethe instantaneous output voltage can be sampled and digitized by anA-to-D converter. Each ternary step can thus be tested in turn duringthe start-up sequence by measuring the output voltage produced. If allternary steps look normal and lamp 204 did not light in the process, theconclusion is that all H-bridge transistors are operating normally withneither short nor open circuits, giving a very low no-load current. TheH-bridges are then operated to generate the normal step approximation toa 120 volt rms sine wave. Again assuming that lamp 404 remains unlit,the inverter is now generating the desired 60 Hz output with no abnormalno-load current. The main power relay (205) is then closed, which shortsout the lamp and start-up relay to allow high currents to flow when theAC load is connected. Note that relay 205 closes only when there is zerovolts across its contacts and zero current through it. This permits itto be a very much lower cost relay than one with sufficiently robustcontacts to make or break full load many times. 60 Hz generation is thentemporarily suspended by controlling the four ternary H-bridges to theopen circuit condition so that the AC output relays may also be closedwith no voltage across them and no current though their contacts. 60 HZgeneration is then resumed to deliver AC power to the load, and if nocurrent overload is detected, the inverter has successfully executed asafe start-up. Both the main power relay and the AC output relays derivetheir coil power from the 13.33 volt output of the 60 KHz inverter, sothat any failure of the DC-DC converter forces those relays to the opencondition. Moreover, the auxiliary PSU is diode-protected such that itwill neither power-on nor be damaged by connecting the DC input withreverse polarity. The start-up relay cannot therefore be operated if theDC input is of the reverse polarity, thus protecting polarizedelectrolytic capacitors and the H-bridge switches from accidentaldamage.

Power down of the inverter can be initiated by a user OFF switch or bydetection of any of the following fault conditions, which are notnecessarily exhaustive:

-   -   Over- or undervoltage at the DC input;    -   Overcurrent from the DC supply;    -   Excessive AC current;    -   Overtemperature on the heat sink;    -   Lamp 204 becoming lit; or    -   Abnormal AC output voltage waveform.        The above fault conditions may be arranged to cause        substantially instantaneous cessation of inverter operation.

A normal Power down occurs in approximately the reverse order topower-up. First, commutation of the ternary H-bridges ceases and theyare switched to the open circuit condition. The AC output relays 131 arethen opened, which occurs with zero volts across the contacts and zerocurrent through them. The main power relay (205) is then opened, whichoccurs with no current flowing through it and zero volts across itscontacts. Finally, the start-up relay 203 is de-energized, whichconnects the reservoir capacitance through the lamp to ground,discharging it. The 60 KHz inverter continues to operate while thereservoir capacitors are discharging, but is deactivated after a fewseconds. The control circuit can check whether lamp 204 lit momentarilyduring the capacitor discharge sequence, as it should. The inverter isthen ready to begin a power-up cycle once any fault condition iscleared. Automatic restart attempts can be programmed to occur up to aspecified number of retries, if desired.

FIG. 12 shows the ternary approximation of the sine wave output from theinventive load converter. A feature of this invention is that all thevoltage steps are of approximately the same size, but have a varyingdwell time according to how fast the sine wave is changing. This gives awell-spread approximation error spectrum with no dominant harmonics.FIG. 13 shows the amplitude of each harmonic of 60 Hz up to 16384×60 Hz,which is in the middle of the medium waveband.

The Medium Wave frequency band begins at about half way from center,where the spectral energy in any 9 KHz AM radio channel is seen to be 78dB below the main 60 Hz component. For a 5 kW (+67 dBm) converter, thisis a level of −11 dBm in the 9 KHz bandwidth of an AM radio. This isstill a high level of radio noise, and interference with radio and TVreception from Medium Wave all the way up to VHF TV frequencies has beenobserved when using prior art converters. Such interference is called“inverter hash” and has been known since the days or rotary converters,or dynamotors, where it was due to commutator/brush noise. Thus, despitethe good sine-approximation evident in FIG. 12, the sheer power of theconverter requires that additional hash suppression be used whenoperating close to sensitive radio receivers. Suppression of radiointerference shall be at least sufficient to meet FCC part 15specifications, but greater suppression is often desirable.

FIG. 10 shows more detail of a complete load converter, including RFIfilters at both the DC input (200) and the converter output (131). TheRFI filters are required, as the output will be distributed around thehouse wiring in a residential application. A suitable 2^(rd)-order LCfilter is shown in FIG. 8. FIG. 14 shows the reduction in medium-waveinterference to 98 dB below the inverter output when using the filter.This additional 20 dB of suppression of inverter hash substantiallyeliminates interference at medium wave and above with radio and TV inthe same location.

As discussed further herein in relation to common mode filtering on theDC side, RFI filters preferably include damping resistors as well ascapacitors and inductors, to ensure that no high-Q resonances occur atany frequency when connected to arbitrary house wiring and appliances.This is the purpose of C2 and R1 in FIG. 8. These components are chosenalso to minimize ringing and overshoot on no load.

FIG. 9 emphasizes the difference between a safe RFI filtering circuit(9B) and an unsafe RFI filtering circuit (9A). Unfortunately, the unsafecircuit of FIG. 9A is in widespread use on the input power connectors ofPersonal Computers, VCRs, and the like, and electronic technicians quitecommonly experience shock from laboratory benches due to the use of suchfilters in laboratory test equipment when the ground is temporarilylost, as can happen accidentally or deliberately in a dynamic R&Denvironment for various reasons. It can also result in inadvertentdamage to expensive equipment upon accidental or deliberatedisconnection of the equipment ground.

With the prior art circuit of FIG. 9A, should the ground connection tothe equipment fail, the line voltage will be divided in two by thepotential divider formed by the filter capacitors, and the chassis orcase of the equipment will become live to the touch, at about 60 volts60 Hz AC (in the US) or 120 volts 50 Hz (in Europe). The current islimited when the filter capacitors are small, e.g., 0.01 uF, but canstill be felt as an electric shock, and is very unpleasant if theequipment is a medical device attached to a catheter, for example.Moreover, when several pieces of equipment are used together, as in arack or a lab bench, the net current can reach more dangerous levels.For an entire household, the typical ground current that can be measuredflowing down the grounding rod is often as high as 50 mA, arising fromall installed appliances such as TV sets, computers, VCR's, washingmachines, etc.

FIG. 9B shows the safer form of RFI filter. In FIG. 9B, the filteringcapacitors decouple the hot line to the neutral line, not the equipmentground as in FIG. 9A. The neutral line is then decoupled to theequipment ground with another capacitor. In this way, should the groundbecome disconnected, the chassis or case of the equipment is onlyconnected via a capacitor to the neutral line, not the hot line, therebyavoiding a safety hazard. It would require both the ground and theneutral wire to fail while the hot line remained intact, in order tocause the case or chassis of the equipment to become live when using theRFI filtering circuit of FIG. 9B, which is therefore a saferarrangement.

For the reasons given above, the output RFI filter comprises a seriesinductor in the 120-volt AC hot leg output, and capacitors C1 and C2connect to neutral, not equipment ground. Furthermore, the neutraloutput is decoupled to equipment ground through the neutral outputisolating relay contacts. In this way, when the output relays are open,there is no path to ground for the inverter, battery or array circuits,not even through the 1 uF neutral-ground decoupling capacitor, thusensuring complete isolation of the DC circuits when the inverter is notoperating.

FIG. 6 shows the common mode hash filter used on the DC input. This alsoreturns the decoupling capacitors to neutral rather than equipmentground for the reasons discussed above. A common-mode filter is used dueto the square wave exported to the DC input lines being in phase on bothterminals. The purpose of this filter is to slow the rise and fall timesof the edges of the 60 Hz square wave in order to avoid exporting radiointerference to DC equipment such as the solar array and battery. The DClines from the array may be protected against induced static duringthunderstorms by means of gas discharge surge arrestors, and it isdesired to avoid peak voltages due to filter overshoot under normaloperation that could cause premature ignition of the surge arrestors.FIG. 7 shows the transient response of the common mode hash filter ofFIG. 6, after optimization of component values for minimum overshoot.

FIG. 2 shows a bidirectional DC-DC converter for providing variousfloating DC output voltages such as Vdc/3, Vdc/9, and Vdc/27 using a DCsource of Vdc volts.

The DC-DC converter uses a high switching frequency such as 60 KHz,enabling the transformer to be much smaller than would be required for60 Hz operation. The transformer in FIG. 2 has four windings, which haveturns ratios N1:N2:N3:N4 in proportion to the voltage ratiosV1:V2:V3:V4. For the purposes of explanation, each winding has beenshown as a center tapped winding with the center taps connected to thepositive terminal of the DC inputs or outputs and the ends of thewindings connected to the drains of N-type MOSFET pairs. Power MOSFETsgenerally comprise a drain-to source body diode which is shown as partof the transistor.

One of the transistor pairs, for example TR1 a and TR1 b, is commutatedat 60 KHz and a voltage V1 is applied to that input. The othertransistor pairs are commutated in synchronism and form synchronousrectifiers. For example, TR2 a is switched on when the transformerwinding end to which its drain is connected goes negative, therebytransferring the negative voltage to the −ve terminal of the V2 output.Likewise TR2 b is switched on when its drain goes negative, and thedrain of TR2 a goes positive, TR2 a is switched off when its drain ispositive and thus does not pass current. Thus a DC voltage V2 isproduced at the V2 output which is N2/N1 times V1, where N2 and N1 arethe numbers of turns on the respective transformer windings. It can beseen that the input circuit for V1 is indistinguishable from any outputcircuit. Thus any one of the transistor pairs can function either as aninverter commutating a DC input or as a synchronous rectifier producinga DC output. Thus power flow can be from any port to any other portdepending on whether the port is sinking current or sourcing current.This is important to the operation of the inventive load converter aspower flows in the reverse direction whenever a ternary digit has theopposite sign to the instantaneous output voltage, or if the AC outputcurrent is not in phase with the AC output voltage due to the non-unityload power factor of a reactive load.

In a presently preferred implementation of the converter, the commutatorused to connect the 120 volt supply to the transformer is a fullH-bridge, eliminating the primary center-tap and simplifying thetransformer. The 40 volt, 13.33 volt, and 4.44 volt windings are howevercenter tapped and connected to synchronous rectifier MOSFET pairs asshown in FIG. 2. Using a transformer center tap is simpler than using afull H-bridge synchronous rectifier, as each synchronous rectifier canshare a gate driver supply with the H-bridge it powers, since theirsources are common. Moreover, the transformer can be divided into two orthree transformers with their non-center-tapped primaries paralleled, inorder to obtain convenient sizes and avoid wiring concentration. In apresently preferred implementation, a first transformer has a 120 voltprimary and a center tapped 40-0-40 secondary, while a secondtransformer has a 120 volt primary and center tapped 4.44-0-4.44 and13.33-0-13.33 volt secondaries. Providing the 40 V supply from a firsttransformer and the lower voltages from a second transformer results inidentically-sized, small transformers.

In designing a DC-to-DC converter, a trade off must be made betweenswitch losses, copper loss in the windings, and hysteresis loss in thecore. Using a higher frequency gives fewer turns of thicker wire, butcore and switch losses increase. In the 60 KHz region, skin-effect issignificant for smaller wire than 23 AWG, so Litz wire comprising manystrands of 23 AWG is used to reduce copper loss.

Core loss persists even when the output power is zero, which can be asignificant contributor to standby (no-load) current. In the inventiveload converter, standby current due to core and switching losses in theDC-DC converter is reduced by use of a novel waveform. Firstly, the 40,13.33, and 4.44 volt outputs of the DC-DC converter are provided withlarge reservoir capacitors. This has the first benefit that current inthe transformer windings is proportional to the mean DC current of eachoutput, rather than the peak current, thereby reducing copper losses inthe windings. A second benefit is that the reservoir capacitors cansupply the low currents required for light loads for at least one ormore cycles of the 60 KHz switching frequency, enabling switching cyclesto be periodically omitted. For example, if the load current is only1/10th the peak output capability, then every alternate DC-DC converterswitching cycle can be omitted and all the transistors of FIG. 2 areheld in the off state during the omitted cycle. In this way, core andswitching losses are halved at 1/10 maximum power output. At even lowerpower outputs, for example 1/100th the maximum, it may be chosen to omit9 out of 10 DC-DC converter switching cycles, which reduces the core andswitching losses even further. Upon detecting an increase in loadcurrent or reduction of any voltage, the number of omitted switchingcycles may be instantaneously reduced in anticipation of the need tosupply greater current to the reservoir capacitors.

FIG. 21 exemplifies the novel DC-DC converter waveforms. FIG. 21(a)shows the DC-DC converter switching waveforms when operatingcontinuously with no omitted cycles. FIG. 21(b) shows the switchingwaveforms when alternate switching cycles are omitted, and FIG. 21(c)shows omitting three out of every four switching cycles. This method ofreducing switching losses at light load is different than merelyreducing the switching frequency. If the switching frequency werereduced instead, the volt-time integral into the transformer on everyhalf cycle would increase, increasing the peak flux density andpotentially causing core saturation. However, keeping the positive andnegative half cycles the same length in microseconds while omittingcycles does not increase the flux density.

An alternative waveform for reducing switching frequency would be tospace the on periods of TR1 b TR4 b midway between the on periods of TR1a TR4 a. However this has some consequences for transformer andcontroller design and is not the preferred method. The presentlypreferred method of omitting complete cycles has the advantages that thetransformer flux is reset to zero after each non-omitted, completecycle, and that all transistors can use the same control waveforms.

An isolated positive pulse or half cycle followed by a negative pulse orhalf cycle may be referred to as a “doublet.” The inventive power-savingwaveform for light loads may therefore be described as driving the DC-DCconverter with doublets, the spacing between successive doublets beingincreased as the current demand reduces.

Power transformers exhibit an “in-rush current” phenomenon, which occurswhen power is suddenly supplied to the transformer at a voltage zerocrossing. The flux in the core rises to a maximum during the first halfcycle, and then falls to zero during the reverse-polarity half cycle.However, that implies a DC bias to the flux, which therefore swingsbetween zero and twice the normal peak. This DC bias decays due towinding resistance after several cycles, but meantime may cause coresaturation and greatly magnified in-rush current. To prevent this, it isadvantageous to modify the waveform of the doublet by including the lastquarter-cycle of the previous (omitted) cycle and the first quartercycle of the following (omitted) cycle, thus preventing the flux densityin the core from exceeding its normal steady-state value. The modifieddoublet and the attendant core-flux waveform are shown in FIG. 25. It isknown from the art of solid state relays to employ peak switching of ACvoltages into inductive loads precisely for avoidance of in-rushcurrent, as just described.

Since high-frequency transformers must operate at much less thansaturation flux density in order to reduce core loss, typically ataround 100 to 200 milliTeslas, it may not be necessary to use the morecomplex modified doublet waveform of FIG. 25 when omitting cycles. Thechoice between the waveforms of FIGS. 21 and 25 should be made independence of the DC-DC transformer design and the properties of themagnetic material used for its core.

A 7.2 kilowatt single-phase standalone inverter has been designedaccording to the above principles using a single 4-layer printed circuitboard (PCB) of size 12″×14″ for all higher-power and high-currentcomponents and one smaller 4-layer PCB for microprocessor and controlfunctions. The power board uses a “maximum-copper” layout, in whichheavy-current conductors are of the widest possible dimensions, formedthrough removing copper in narrow strips to isolate different circuitnodes, thereby leaving the maximum amount of copper, rather than theconventional PCB layout technique of defining narrow tracks of copperwhere conductors are required and etching away everything else. Powertransistors requiring heat-sinking are contained within a 14″×5″ sectionin the middle of the board and have their leads soldered to the board onthe component side, so that no leads or solder bumps protrude to theother side, which may then be bolted flat to a metal heatsink formingpart of the case of the inverter. The inverter, as with other equipmentdesigns described herein, may be designed to be mounted flush with thewall covering (e.g., SHEETROCK™) in the 14″ gap between two studs at thestandard 16″ spacing in US residential wood frame construction. Theouter gridded surface of the heatsink is inside the wall, and louveredvents may be placed above and below the inverter to provide convectiveairflow as necessary, with the gap between studs acting as a chimney toenhance convection. A thermistor on the heatsink protects the inverterfrom over-temperature in the event of use at maximum power for prolongedperiods with restricted airflow.

While the above-described load converter provides some proportion of ahousehold or business electricity needs and provides a desirable batteryback-up facility, the timing of the demand is not necessarily correlatedwith solar irradiation, which is the purpose of storing energy in abattery for future use. Currently, lead acid batteries are the onlyeconomic storage solution, but this may change if electric vehiclebattery research and their large production volume succeed in reducingthe cost of other battery types. In general, storage batteries, and leadacid batteries in particular, survive only a finite number ofcharge-discharge cycles before replacement becomes necessary. To prolonglife, it is desirable to avoid 100% depth of discharge on a regularbasis, and to allow normally only 20% depth of discharge, except duringthe emergency situation of a utility outage. Using 10, 12-volt, 100ampere-hour, deep-cycle batteries, the energy storage available during autility outage would thus be 12 kilowatt-hours, which will run essentialhousehold appliances for a considerable period. Only 20% or 2.4 kilowatthours should be used regularly however. Thus it is only possible todefer 2.4 KwHrs of consumption of solar energy to a later period of theday without using a much bigger battery. Some of this supply-demandtiming mismatch can be resolved by using smart devices to start loads,such as a dishwasher, according to a timer, or even according to solarillumination. Another possibility is to divert solar energy in excess ofbattery charging needs to HVAC or other diversionary load. However,there is a limit to how much solar array energy can be absorbed by ahousehold immediately due to the very varying demand and supply curves.To improve the supply-demand match, solar energy can be diverted to theutility grid where the demand curve is averaged over millions of users.This is the purpose of a grid inter-tie inverter.

Before describing the inventive grid-tie inverter in more detail, asolar combiner is described that assists smart energy management in acomplete installation. The circuit of solar combiner 700 is shown inFIG. 27. A number of DC inputs 700-1 to 700-n may be connected to theterminals of individual solar panels or different strings of solarpanels. The inputs are isolated and have separate −ve and +ve terminalsfor wiring to the solar array. Each pair of input wires is connected tothe input of a respective per-string circuit 710-1 to 710-n. Eachper-string circuit comprises fuses 701 a, 701 b on both the positiveline and the negative line to protect wiring from fault conditions. EachDC input line then proceeds through snubber circuit (702, 703, 704) andblocking diode 705 to DPDT relay 706. Blocking diodes 705 preventreverse current flow to any string or panel exhibiting lower voltagethan the other strings, for example when a panel is shadowed. Whenproperly selected, blocking diodes have a voltage drop of around 0.6volts, so there is a compromise between the diode loss and the currentloss due to shadowing without diodes. For array voltages below about 36volts, it may be more efficient to omit the blocking diodes and tolerateshadowing loss, while it is more efficient to employ them for higherarray voltages. An advantage of blocking diodes is that they alsoprevent blowing a fuse if a string is accidentally short-circuited.Blocking diodes may also be desirable for lower array voltages if asubstantial fraction of the total number of strings can besimultaneously shadowed.

Snubber circuit comprising capacitor 702, diode 703, and resistor 704operates in conjunction with blocking diode 704 to protect the contactsof relays 706, 707 from high in-rush current and arcing on making orbreaking. Consider for example that the load on output 1 comprises alarge capacitor, e.g., the reservoir capacitor on an inverter. Whenrelay 706 is closed to route current to output 1, the load mayinstantaneously appear like a short circuit, pulling the voltage of thepositive line down to the potential of the negative line. This negativegoing transient passes through capacitor 702 and reverse biases diode703, so that the only current that flows is the array short circuitcurrent, which is inherently limited, plus the current flowing throughresistor 704 to discharge capacitor 702. The load voltage then risessmoothly with relay 706 contacts closed and capacitor 702 chargesthrough resistor 704 to the same voltage. When relay 706 contacts opento disconnect the string from the load, the string voltage attempts torise, and the positive going transient passes through capacitor 702 toforward bias diode 703, which therefore caps the voltage rise at about0.7 volts. Thus the relay contacts are only required to break loadcurrent with 0.7 volts across them. In another situation, the load mayalready be charged or powered by other strings when relay 706 closes. Ifcapacitor 702 is not already charged, blocking diode 706 will prevent ahigh back-in-rush current from the load, leaving capacitor 702 to becharged only by the inherently limited array current. A suitablecapacitor 702 is about 220 uF with a working voltage above the highestopen circuit string voltage with which the combiner is intended tooperate.

One positive and one negative output contact from each of relays 706 areparalleled to the DC output bus connected to output 1 terminals 721.Each relay may be controlled, via control port 1, to connect itsassociated string to output 1. Thus the combiner can be remotelycontrolled via control port 1 to deliver the current of 0, 1, 2, . . . ,n strings to output 1. Typically, eight relays (706) would be driven bya relay driver chip located in a controller. The controller may be partof inverter 1000. A suitable relay driver chip is the Texas Instrumentspart number TPL9201. Output 1 may be connected to a battery and a loadconverter. The load converter can monitor the battery voltage andoptionally the battery net current and, via control port 1, direct thecombiner to output more or less current to output 1 by selecting ordeselecting strings in order to keep the battery at an optimum state ofcharge. The load converter can thereby achieve intelligent batterymanagement, by implementing in software different charge regimes such asbulk charge, absorption charge, equalization charge, and float charge,in dependence on the history of the battery state. Alternatively, aseparate charge controller can implement these functions in cooperationwith the solar combiner 700. The advantage of involving the solarcombiner in charge control is that any string not required to achievethe desired instantaneous load inverter plus battery charge current canhave its current diverted to a secondary output 2.

When relay 706 is not selected to output its associated string currentto output 1, the current is routed to relay 707. Relay 707 may beprogrammed to divert the current to a second output bus connected tooutput 2 terminals 722. Output 2 is connected to a load that is of lowerpriority than the load on output 1, as control port 1 must have caused arelay 706 to relinquish current in order for output 2 to receive it. Thesecond priority load can comprise controls connected to control port 2to select any of the strings relinquished by the first priority load, ornone. When control port 2 does not select a string, its output appearson associated string test points which are isolated from all otherstrings and loads, which is useful for maintenance purposes. A bleedresistor 708 is connected across each test point so that, upondeselecting a string for maintenance purposes, the charge on capacitor702 is bled down to a safe value in about 30 seconds. If neither thefirst nor second priority load selects any string, then all strings areisolated from each other and the loads, thereby providing a DCdisconnect function. DC disconnect can be forced using switch 709 localto the combiner, which removes power from all relay coils. Second orfurther switches can optionally be provided at remote locations to forceDC disconnect. For example, a DC disconnect can be provided at aninverter, at the main service entrance of a building, and so forth. Inany case, when the inventive load inverter is connected to output 1 andcontrol port 1, if the inverter ceases inverting for any reason, relaypower will not be supplied to control port 1, and DC from the array willbe disconnected from output 1. Likewise, if a grid-tie inverter isconnected to output 2 and control port 2, cessation of inverting willdisconnect relay power from control port 2 causing all relays 707 todisconnect DC power from output port 2. An inventive load converter andan inventive grid tie inverter may also be coupled such that, if oneceases inverting, the other will also cease inverting. Optionally,relays 707 can be replaced with manual DPDT switches, and then the loadon output 2 automatically receives the current of any string not usedfor output 1, as long as the associated manual switch has enabled it.However, the disconnect function of manual switches cannot beremote-controlled from another location.

Since each per-string circuit 710 is identical, a small sub-board can bedesigned to accommodate the components, and only as many installed in aparticular combiner as there are strings to be combined. Since nocombiner is required for a single string, it is logical to put at leasttwo circuits 710 on a sub-board, and then to install as many sub-boardsas required to reach or exceed the desired combining capacity.Parameters for a typical combiner board are:

-   -   Absolute Maximum string voltage: 380 volts    -   Maximum number of strings per combiner: 8    -   Maximum current per string: 8 amps    -   Total maximum current: 64 amps

FIG. 15 illustrates an inventive single-phase grid inter tie inverter.As in the load converter configuration, the circuitry is simplified bythe use of a floating solar array in which neither terminal ispermanently connected to ground or the neutral line. The floating DCsupply from the solar array is fed through common-mode filter 200 toH-bridge 2010. The four MOSFET power transistors of the H-bridge areable to be turned on and off by opto-isolated controller 2020. Tr(a) andTr(b) connect the positive and the negative of the DC supply alternatelyto the AC output neutral line, which is a grounded conductor. The DC− isconnected to the neutral when the AC output is required momentarily tobe positive, and the DC+ is connected to neutral when the AC output ismomentarily required to be negative. This switching is synchronized withthe utility grid so that the two solar array terminals are alternatelygrounded at a 60 Hz repetition rate. Common mode filter 2000 slows theswitching edges as previously described so as to avoid the export ofhigh frequency interference. The voltage on each solar array terminalrelative to ground thus comprises an average of about 85 volts DC withan 85-volt peak square wave superimposed. For the safety of electriciansor other personnel who may work on the solar system, the solar array andother parts of the DC circuit shall, according to the NationalElectrical Code (2008) be labeled with an indication that both DCconductors are hot. An appropriate wire color coding is red for positiveand black for negative, both of which signify “hot” in an AC context,while identifying positive and negative in a DC context.

When the DC −ve is connected to neutral, the other two H-bridgetransistors TR(c) and TR(d) operate at a high switching frequency, forexample 200 KHz, to connect the DC+ to the AC output with a mark-spaceratio varying in proportion to the desired sinewave current waveform.The on-off switching can for example be a delta-sigma modulationrepresentation of the desired positive half cycle. Thus TR(c) turns onfor a time to cause current to increase in L1. Tr(c) then turns off andTR(d) turns on to catch the flyback from L1. TR(c) and Tr(d) operate inreverse for a negative half cycle, when the DC+ is connected to neutral.The required switching patterns may be precomputed and stored incontroller 2020 memory for one complete 60 Hz cycle, or else synthesizedin real time to create the desired current or to follow the utilityvoltage. C1 further smooths the current from L1 to deliver a sine wavecurrent to the utility. A further RFI filter 2030 may be used to reduceexport of inverter hash which could otherwise cause radio interference.

The inverter of FIG. 15 has in principle a high efficiency whenoperating at normal power levels, but does not operate efficiently atgreatly reduced power levels. One reason for this is that the impedancesof L1 and C1 must be related to the output current and thus valuesappropriate for the maximum current do not provide proper smoothing atmuch lower currents. Moreover, the high frequency switching losses,while being negligible at full power, would be relatively moresignificant at lower powers. Also, this type of inverter must deliver acurrent to the utility at whatever voltage the utility exhibits, incontrast with the previously described load converter which must delivera constant voltage to the load, whatever current the load draws. Forthese reasons, a grid-tie inverter does not make a good load converter,and vice-versa, although it is possible to design an inverter, known asa bimodal inverter, which can be wired up and operated in either mode.

Both types of high-efficiency converter have a number of things incommon however, as well as the inventive principles described by theattached claims: In the single-phase case, the instantaneous powerdelivered to the load or utility is proportional to the square of thevoltage, and therefore follows a sin² (wt) curve which is adouble-frequency raised cosine ranging from zero watts to twice the meanpower. In the load converter case, the battery evens out this demand andallows the solar array to charge the battery at a constant current. Inthe case of the grid-inter-tie inverter however, without the battery,there would be no means to even out the demand, and thus the solar arraywould have to be sized to deliver twice the mean current with consequentunder-utilization at other times in the 120 Hz cycle. To avoid this, asingle phase grid inter-tie inverter requires a substantial inputreservoir capacitor C2 to even out current flow over each 120 Hz cycle,and thus to match it with the constant current available from the solararray.

If the grid-tie inverter delivers 60 amps rms to the utility at 120volts, that is 7.2 Kw, the mean current from the solar array is 7.2 Kwdivided by the DC input voltage of, say 175 volts; that is, 41 amps. Thepeak current required by the inverter is 82 amps. C2 must thereforesupply 41 amps during a positive-going 120 Hz half cycle and recharge at41 amps on the negative-going half cycle. Neglecting the solar arrayoutput resistance, the 41 amp peak cosine current ripple must beabsorbed by C2 without producing a large ripple voltage. For 2 voltspeak ripple, the value of C2 is given by 2Π 120 C2=41 amps/2 volts, fromwhich C2 is determined to be approximately 27000 uF. The size andconstruction of this capacitor is determined more by its ripple currenthandling requirement than by its capacitance, and consists of severalaluminum electrolytic capacitors in parallel. This reservoir capacitorthus operates with a current ripple equal to the full output current,and must therefore be operated very conservatively and well within itsmaximum ripple current capability to prevent degradation. Degradedelectrolytic capacitors operating with high voltages and ripple currentscan overheat and burst causing a big mess, as often happened withantique tube radios, and has happened more recently with grid-tieinverters of the prior art. As will be shown, the inventive three-phasegrid-tie inverter can operate with a much smaller smoothing capacitor,as the current demand from the solar array for the three phase caseconsists of three, double-frequency raised cosine curves spaced 240degrees apart, which substantially cancel each other and produce muchless ripple current.

The grid-tie inverter is self-regulating, in that it attains a level ofoutput current to the utility which allows the solar array voltage torise to the required minimum of √2 times the peak utility voltage, thatis about 170 volts assuming 120 volts at the utility.

A grid-tie inverter has to meet specific safety requirements and inparticular the anti-islanding requirement specified in UnderwriterLaboratories standard UL1741. Anti-islanding refers to means that mustbe employed to prevent the inverter from attempting to back-power aneighborhood during a utility outage, as this could endanger personneltrying to fix the fault. Anti-islanding is achieved by having theinverter controller 2020 derive its timing cues from the utilityvoltage, for example by waiting for a utility voltage zero-crossing totrigger the next switching cycle. If the utility is in outage, leavingone or more utility converters in a neighborhood connected to the grid,they will each wait for the next zero crossing and the frequency willtherefore drift until it is clearly out of limits, at which point theinverter stops and opens the output disconnect relay 2100. The inverteris also programmed to trip out if the voltage at its utility connectionis outside of specified limits, which is also indicative of disruptionof the utility connection. Controller 2020 continues to monitor theutility voltage, and if it returns to within predetermined voltage andfrequency limits and remains there consistently for 30 seconds, theinverter will resume operation. Various integrated circuits have beendeveloped to monitor the grid voltage and frequency and are available onthe market to facilitate the implementation of anti-islanding; forexample, the Analog Devices part number ADE7753.

The presence of an AC component on the array DC terminals has a numberof benefits; for example, it facilitates detection of ground leakagefaults on the DC side. If the inverter is connected to the utility via aregular two-pole AC GFI breaker, any leakage path on the DC side willresult in a 60 Hz imbalance current between neutral and hot on the ACside, thus tripping the breaker. The anti-islanding circuits then detectfailure of the utility input and shut off the inverter, opening theoutput relays. and the combiner relays, effecting an automatic DCdisconnect. The two pole GFCI breaker uses one pole to interrupt the hotleg and the other to interrupt the neutral. The breaker pigtail is usedto provide the ground connection for upstream equipment so that thesensitivity of the breaker is not affected by capacitive currents to thecable sheath or conduit, but only by an unexpected source of groundleakage.

Alternatively, a specific common-mode current transformer can be used onthe DC feed to detect any AC current caused by a ground fault on the DCside. A presently preferred wiring method to connect the inverter to thearray combiner, and to connect each photovoltaic string to the combiner,is insulated, flexible metallic sheathed conduit also known asLiquidtight (metallic). The flexible metal sheath encloses the DCconnections and provides some screening of residual inverter hash. Theinsulating outer sheath on the Liquidtight also prevents accidentalconnection with any other grounding source, such as a water pipe, anddiscourages its unauthorized use as a ground for other appliances orcircuits. The sheath of the Liquidtight is ultimately grounded via theGFCI breaker pigtail, if this method of DC ground fault leakagedetection is employed.

Other benefits in the single phase case of the 60 Hz AC component on theDC conductors concern the specification of relays and fuses. Anydisconnecting device, such as a switch, relay, or fuse, must usually beparticularly designed and specified to break a DC circuit as opposed toan AC circuit. AC is easier to break, because there are two voltagezero-crossings per cycle that will extinguish any arcing rapidly. In thesingle-phase case, the inventive converters alternately ground thepositive and negative DC conductors. Therefore the voltage on either DCconductor is zero for a whole half-cycle, and not just a zero-crossing.Thus it is permissible to use AC-rated fuses in combiner 700 to protectagainst shorts to ground of any array wiring. Likewise, the batteryfuses may be AC rated rather than DC-rated, or either. In thethree-phase case however, DC-rated fuses are required, as the DCconductors do not exhibit voltage zero-crossings or zero periods.

For higher power levels, for example 20-100 kw or above, utilitiesprefer or insist that 3-phase power be delivered equally on all threephases to reduce cable costs and to maintain load balance. In the priorart, a three-phase inverter was constructed using three, synchronizedsingle-phase inverters, and even three separate solar arrays. With theinventive 3-phase inverter design, an integrated 3-phase inverter isprovided that operates from a single solar array. It is even moreeconomic, per kilowatt, to construct the inventive 3-phase inverter thanthe inventive singe-phase inverter, due in large part to the eliminationof the need for the large input reservoir capacitors.

FIG. 16 shows the principle of a 3-phase sine wave inverter according tothe invention. The DC input is applied to three half-bridges that canconnect either DC+ or DC− through an LC smoothing filter to each of thethree-phase outputs a, b, and c. The three phase utility connection canfor example be a 120/208 connection with neutral, i.e., the utilitytransformer has a Y-configured secondary while most likely having aΔ-primary. Other three-phase utility configurations are possible. Forexample, a Δ-secondary with no neutral can be used. However, theY-connection is presently preferred so as to allow the RFI filtercapacitors to decouple RF interference to neutral rather than ground,for aforementioned safety reasons Also, some means would be needed whenusing a Δ-secondary to prevent the system floating to an arbitraryoffset voltage—for example leak-resistors to ground from all threephases. The inventive 3-phase inverter can also be adapted to operatewith a 120/240/208 volt system having a so called 208-volt “high leg.”In Europe, a common 3-phase voltage is 240/416, which results in aninverter adapted to European voltages supplying twice the power (e.g. 40kW) for the same current.

FIG. 23 shows the inverter voltage waveforms at the output to theutility and on the input DC+ and DC− lines. The half-H-bridges operateaccording to the following logic:

(1) If Phase(a)>0 v and Phase(b) and Phase(c)<0 v then connect DC+ tophase(a).(2) If Phase(a)<0 v and Phase(b) and Phase(c)>0 v then connect DC− tophase(a).(3) If Phase(b)>0 v and Phase(c) and Phase(a)<0 v then connect DC+ tophase(b).(4) If Phase(b)<0 v and Phase(c) and Phase(a)>0 v then connect DC− tophase(b).(5) If Phase(c)>0 v and Phase(a) and Phase(b)<0 v then connect DC+ tophase(c).(6) If Phase(c)<0 v and Phase(a) and Phase(b)>0 v then connect DC− tophase(c).

Operating the switches according to the above logic causes a common-modeAC signal to appear in-phase on DC terminals with a frequency of threetimes the AC output frequency of the inverter, which is 180 Hz for anoutput frequency of 60 Hz. The 180 Hz common mode signal is almost asine wave, and requires less filtering than the 60 Hz square wave of asingle phase inverter. It is also possible to construct split-phaseinverters according to the invention, the split phase inverter providingtwo AC output hot leg terminals with relative 180 degree phasing.Alternatively, a two-phase inverter can be constructed according to theinvention that provides two AC output hot leg terminals having a90-degree relative phasing. In the latter case, a Scott-T transformer ofthe prior art can be used to convert the two-phase output to athree-phase output, if so desired.

When a first switch connects a DC input line to one of the phases, theother DC line is simply the DC supply voltage away from that phase involtage. That other DC line is then chopped at high frequency by otherswitches to generate, after hash-filtering, the best approximation tothe desired voltage difference to the other phase. In particular, themark space ratio of the chopping action rises to 100% ON at the peak ofthe phase-to-phase voltage. The phase-to-phase voltage is 120√3=208 vrms for a 120/208 three-phase service, and the peak is a further factorof √2 higher. Thus the solar array is loaded down to a DC voltage of120√6=294 volts, assuming the AC voltage output is 120 volts rms perphase. With this voltage, the DC+ and DC− lines are seen to execute anear-sine waveform with an amplitude of 45.47 volts peak to peak and ata frequency of three times the AC output frequency. This 180 Hz waveformthat is superimposed on the DC lines is not quite a sine wave, as may bedetermined by computing its spectrum, but is sufficiently free of highfrequency harmonics that the common mode filter needed for a three-phaseinverter may be simpler than in the single-phase case. Moreover, theinstantaneous power delivered to the load is a flatline in the 3-phasecase, and therefore the current from the DC supply is also a flatline,and not a 120 Hz waveform, as was the case with single phase. Thereforein the 3-phase case, capacitor C2 does not have to smooth out afull-current 120 Hz ripple waveform, as it was required to do in thesingle phase case, and a smaller value suffices to prevent export ofinverter hash to the DC circuits. The ripple is low only when connectedto a three-phase utility having substantially equal voltages on eachphase and substantially the correct 120-degree relative phasing. Theutility is normally guaranteed to maintain these parameters within quitetight limits, as otherwise three-phase motors can be damaged; however,fault conditions can arise, such as loss of a phase, which requiresmonitoring. A simple method to monitor the utility phase and voltagebalance is to connect equal value resistors from each phase to a commonpoint, which should then be at zero volts AC relative to neutral.Detecting the residual voltage at this point provides a measure ofutility phase and voltage balance. If the measured balance value isoutside of a threshold value, a utility input fault condition isdeclared and the inverter powers down and the output relays are opened.Three small, fused control transformers remain connected to the utilityto monitor resumption within correct limits, and also supply power tothe control microprocessor and other control circuits even in theabsence of solar power at the DC input. Thus in contrast to the loadinverter, the grid-tie inverter requires a utility input in order tooperate. Without a utility input, the grid-tie inverter cannot supplyrelay control signals to combiner and DC disconnect unit of FIG. 27, andthus the DC input is disconnected at source.

The AC outputs of a single-phase or 3-phase grid-tie inverter are alsopreferably protected against voltage spikes on the utility connection.This can be done by use of gas-discharge tube surge arrestors, forexample. Such devices have instantaneous current-sinking ability ofthousands of amps, and can tolerate such currents long enough to trip a60-amp breaker, should the over-voltage transient persist. These shouldpreferably be connected between line and neutral, but in the case of adelta-connected 3-phase service with no neutral, they can be connectedbetween line and utility ground and/or line-to-line.

In summary, a 3-phase inverter according to the invention delivers threetimes the power output of a single-phase inverter using only about thesame number of components, and uses a smaller capacitor on the DC input.A 3-phase grid-tie inverter of 21.6 Kw capacity is therefore only of thesame order of complexity and cost as a 7.2 Kw single phase loadinverter.

There are many subtleties in designing such an inverter to back-feedpower to the grid. FIG. 26 shows a vector diagram pertaining to avoltage generator which is presumed to have an inductive outputimpedance and a vector output voltage of V1, connected to a utility gridof vector voltage Vo. The vector difference between (V1−Vo) between thegenerator voltage V1 and the utility voltage V2, divided by theinductive output reactance wL, determines the current flow to the grid.The current lags the voltage across the inductor L by 90 degrees;therefore in order for the current to be in phase with the grid, i.e., ahorizontal vector, the difference voltage (V1−Vo) must lead by 90degrees, i.e., be a vertical vector. Thus either the generator phasemust be advanced from φ1 to φ2, or else the voltage must be reducedbelow V1 in order to rotate (V1−Vo) to the vertical position, or evenboth the phase and the voltage must be adjusted.

In the case of the grid-tie inverter, the generator voltage V1 isproportional to the array voltage and the array voltage is inverselydependent on the current taken from the array, which is proportional tothe component of the current in phase with the utility, i.e., theprojection of the current vector on the horizontal axis. Moreover, sincewL is by necessity a very small reactance, (V1−Vo) is a very shortvector, of the order of 1 to 2 volts, and the sensitivity of the currentphase to changes in array voltage and thus V1 is very high. It may bededuced that changes in the generator phase φ cause large changes incurrent magnitude while changes in array voltage and thus V1 causechanges in current phase, but, to a first order, not its magnitude. Thusproper operation of the grid-tie inverter requires controlling thegenerator phase advance relative to the utility to adjust the currentmagnitude until the array is operating at a desired point, i.e., themaximum power point (MPP). One strategy is thus to advance the inverterphase (which increases the output current) if the array current is toolow and thus the array voltage is too high, or conversely to retard thephase if the array voltage is too low because the current is too high. Asimplified strategy is to control the phase of the inverter such thatthe phase of the current is in phase with the utility, i.e., to controlthe inverter phase such that the voltage across the inductor L has aphase that leads the utility phase by 90 degrees. If this results in toolittle current, such that V1 increases, the result will be that thecurrent will lag the utility, and the generator phase shall then beadvanced, increasing the current and reducing V1. Conversely, if thecurrent is too high, causing V1 to reduce, the current phase willadvance over the utility phase and thus the inverter phase will then beretarded to bring the current into phase with the utility. Maximum powerpoint tracking is then approximately achieved at the same time assynchronizing the inverter to the grid.

The phase of the voltage across the inductor L1 (FIG. 15) or La, Lb, Lc(FIG. 16), which are the switch-mode smoothing inductors, is simplysensed by adding a secondary winding thereto, or else by adding asecondary winding to the following RF hash filter, and feeding thesecondary voltage to a zero-crossing phase detector. The secondarywinding senses the derivative of current. The secondary voltage may thusfirst be integrated to obtain a 60 Hz current-related signal. The phasedetector simply comprises logging the count in a master clock counter atwhich a current zero crossing occurs and determining if it was early orlate. This shall be done for all three phases in a 3-phase system, andeither a mean taken, or else each phase can be separately controlledfrom its own sensor coil.

The three phase inverter of FIG. 16 is a simplified circuit, and has thecharacteristic that the DC input is not switched to connect directlywith any AC output terminal, but rather connects it through thesmoothing inductors La, Lb, or Lc to one of the output terminals. Thishas the result that some of the high frequency switching waveform istransferred to the DC input, which would require a more complicated DCinput filter. FIG. 24 shows a switching arrangement to avoid this and toensure that, at the appropriate time, a DC input line that is selectedto connect to an AC output terminal is directly connected to that outputterminal, bypassing the associated smoothing inductor La, Lb, or Lc.

Referring to FIG. 24, in period 1 of the three-phase cycle, Phase a andPhase c are positive or zero while Phase b is negative. According to theabove-described switching logic therefore, the Phase b output terminalshall be directly connected to the DC negative input line. This is doneby turning on TR4 b to conduct. Just prior to period 1, Tr2 b wasexecuting a switching cycle with varying duty factor, the duty factorhaving just reached 100% at the beginning of the period. Thus TR2 b isalso left conducting when Tr4 b is switched on, and shares the loadcurrent with Tr4 b. At the end of period 1, phase b and phase ctransition to negative while phase a is positive. Accordingly, thephase-a output terminal shall be directly connected to the DC inputpositive line. This is done by turning on Tr3 a. Just prior to period 2,Tr1 a had been switching but had reached a 100% duty factor. ThereforeTr1 a is left 100% ON to share the load current with Tr3 a. At theactual boundary between two periods, for example at the boundary ofperiod 2 and period 3, the switches operate according to the “Tarzanprinciple”—that is, do not let go of one vine until grabbing hold of thenext one. Thus for an instant, Tr1 a and Tr3 a are connecting the DCpositive input line to the phase-a output terminal while TR2 c and Tr4 care connecting the DC negative line to the phase-c output terminal. Inthis way, the DC input is always anchored by either its negative line orits positive line to one of the three phase output terminals.

It can be mentioned that, to obtain proper relative phase of theswitches taking into account the delay through inductors La, Lb, and Lc,the switching waveforms of Tr1 a, Tr2 a, Tr1 b, Tr2 b, Tr1 c, and Tr2 cmay be slightly time-advanced relative to the switching waveforms of Tr3a, Tr4 a, Tr3 b, Tr4 b, Tr3 c, and Tr4 c. This time advance correspondsto the phasing of the generator relative to the utility voltagediscussed in relation to the vector diagram of FIG. 26. Thus the phaseadvance of the waveforms for switches Tr1 a, b, c and Tr2 a, b, c isadaptively controlled to make the AC output current lie in phase withthe utility voltage while switches TR2 a, b, c, and TR4 a, b, c operatein a fixed relationship to the utility voltage.

It can be mentioned that the testing of high power converters to confirmefficiency and adequate heat dissipation at maximum load can consume aconsiderable number of kilowatt hours. It is also difficult to measureefficiencies in the 98-99% region by measuring the input DC power andoutput AC power with different instruments. A 1% measurement inaccuracycan lead to the erroneous conclusion that an inverter is more than 100%efficient, or that it is less efficient than it really is. A presentlypreferred method to test an inverter is therefore to rectify the ACoutput with a rectifier circuit especially designed to consume asinusoidal current in phase with the voltage, and to feed the DC backinto the inverter input. The inverter DC source need then only supplythe difference between the input and output power, i.e., the amount ofpower wasted in inefficiency, plus the rectifier circuit inefficiency.This is both a more accurate and more cost-effective way of testinginverters.

The grid-tie inverter described herein may be operated in reverse totransfer energy from an AC source to a DC load. It simply requiresprogramming the phase control circuit described above to ensure that thephase of the voltage across the series smoothing inductor is 90 degreesretarded with respect to the AC voltage. Power will then flow from theAC to the DC circuit. As illustrated in FIG. 26a , for rectifier mode,the amount of phase retard should be controlled to produce the desiredamount of AC load current and thus rectified DC current while thevoltage V1 is controlled by adjusting the switching waveform duty factorto make the AC current in phase with the AC voltage. The same circuitarrangement can thus be used with an alternative control regime to makea rectifier which presents a linear load of low power factor to the ACsupply. This is useful for inverter testing as described above, and isalso useful for high power battery chargers for electric vehicles whereit is desired that they should present a linear load to the utility.

FIG. 18 shows the block diagram of an electrical installation employingthe first load inverter implementation to provide a household withsolar-generated AC power. A solar array comprising one or more stringsof solar panels is installed on a south-facing roof for example, and anyexposed metallic parts other than its electrical terminals shall bereturned to ground via a grounding conductor to give some degree ofprotection against thunderstorm transients. Each string of panelsconnects to solar combiner 700 therefore using three conductors—the DCpositive, the DC negative and the grounding conductor. The preferredwiring method between a string of solar panels and the solar combiner isto use Liquidtight flexible metallic conduit (LFMC) to carry thepositive and negative lines of at least one string as well as an arraygrounding wire. The metallic sheath of the Liquidtight should also bebonded to the array frame (if metal), and the solar combiner case, thuseffectively providing a coaxial connection. Such a coaxial connection isa superior method of preventing thunderstorm transients entering intothe signal and power paths. The 2008 National Electrical Code suggestedthe use of a separate grounding rod for the array when the array wasremote from other equipment. However it was recognized that this couldbe inadvisable and that requirement was removed in the 2011 version ofthe Code. For installing systems using devices according to thisinvention, all grounding shall be routed back to the common premisesgrounding electrode system. This may consist of more than one groundingrod or electrode, however all grounding electrodes must be connectedtogether on the load center side of the inverter and no other groundshall be connected to the installation on the battery or array side ofthe ground leak detector.

As already described, solar combiner 700 selectively combines thecurrents from multiple strings of solar panels according to relaycontrol signals presented to its control port 1. Standalone inverter1000 is connected to control port 1 via a 9-conductor cable, for examplean 8-conductor-plus-braid Cat 5 cable, or a DB9 serial port cable, andcontrols the string selection in order to maintain the voltage ofbattery 500 within desired limits. To avoid prejudicing operation of theground leak detection function, the braid of the Cat 5 cable or groundreturn of a DB9 cable must not be connected to the ground or any part ofthe DC circuit inside the combiner. The relay coils provide this desiredisolation. One method, which requires monitoring only the batteryvoltage, progressively switches out strings of panels when the batteryvoltage approaches a target value. For example, suppose battery chargingis operating in the float charge regime where the battery voltage shallbe maintained at approximately 135 volts for 60 lead-acid cells inseries. If the battery voltage is less than 120 volts, all strings areselected to charge it. As the voltage approaches 135, strings areswitched out depending on the rate of approach, which is determined bybattery-voltage monitoring circuit that can be contained withincontroller 202. For example, if the voltage is 126 volts and rising atthe rate of 0.2 volts per second, it can be predicted that the voltagewill reach 135 volts in 45 seconds. Thus in five seconds, one string isswitched out. If the battery voltage one second later is less, one morestring is switched back in. If on the other hand the battery voltagecontinues to rise such that it will reach the target voltage in lessthan 45 seconds, one more string will be switched out. The foregoingdescription of float charging is merely intended to be exemplary of allpossible methods of controlling battery charging when only monitoringbattery voltage. Preferably, the battery charge controller would also beable to monitor net battery charge current. This requires a currentsensor at the battery such as a very low resistance current shunt (notshown) with a pair of dedicated wires back to the DC-AC converter 1000,where the battery charging logic and control resides.

The combined current from the selected strings is routed from solarcombiner 700 to battery 500, again using preferably LFMC to maintain acoaxial path. The trade size of Liquidtight shall be sufficient for thegauge of conductors needed for the total current of all strings, plus anequipment grounding conductor used together with as the Liquidtightmetal sheath to transport the equipment ground. Battery 500 shall behoused in a suitably ventilated enclosure to dissipate any hydrogenout-gassing safely to the outside. The housing may include a perforatedmetal or foil screen which then should be bonded to the Liquidtightsheath and the equipment grounding conductor. The 2008 NationalElectrical Code leaves it optional whether the battery wiring includesfuses. Since a large storage battery is a source of potentially hugefault current in the event of a short in the wiring, it is highlyrecommended that fuses be installed as close to the battery as possible.At least two fuses are required for this purpose. For example, one fusemay be inserted in the positive connection from the combiner 700 and asecond fuse is then inserted in the positive connection to the inverter1000. A common fuse in the battery connection must not be used, as inthe event of its blowing, the array would be left directly connected toload inverter 1000, which is an unstable arrangement. Four fuses (550)may also be used, connecting one in each of the positive and negativeconductors to and from the battery, as shown in FIG. 18.

Liquidtight flexible metallic conduit 400 or other metallic conduit isused to route the wires from battery 500 through the ground leakdetector comprised of items 800, 801, and 802 to inverter 1000. Theconduit and its enclosed positive, negative, and ground conductors passin their entirety through a large, high-mu ferrite toroid 800. Asuitable toroid is Magnetics Inc. part number 4916 in W material with amu of 10000. The inner diameter of this toroid is 33 mm. Thisaccommodates trade size ¾″ LFMC. This size LFMC accommodates three #4AWG THWN-2 conductors, which are good for 95 amps continuous. Thegrounding conductor may be of a smaller gauge, for example #6 AWG.Toroid 800 is wound with about 100 turns of relatively fine wire, suchas #20 AWG, as it does not have to pass any current. This forms a 1:100voltage step-up transformer between the net current flowing in theconduit and its conductors, which should be zero if there is no groundleak on the battery/array side of the toroid. The 100-turn secondary hasan inductance of 118 mH while the effective 1-turn primary has aninductance of 11.8 uH. A 60 Hz imbalance current in the conduit of 6 mAwill cause a primary voltage of 0.0267 mV and a secondary voltage of2.67 mV. The sensitivity may be increased by adding capacitor 802 toroughly resonate the secondary inductance of 118 mH. A 60 uF capacitorresonates the inductance and also suppresses signals at other than 60Hz, such as medium wave radio stations, or nearby amateur radiotransmissions. A load resistor of 220 ohms keeps the Q-factor of theresonant circuit below 5 and thus insensitive to small componentvariations. With a Q of 5, the induced voltage due to a ground leakcurrent of 6 mA rises to 13 mV. An amplifier inside inverter 1000amplifies this voltage and compares it to a threshold. If it exceeds athreshold corresponding to 6 mA, the inverter executes a shutdown,controlling the combiner to the DC disconnect state and opening both theinverter AC output relays and the DC input power and start-up relays.This completely isolates the array from the battery and ground, and thebattery from the inverter and ground, thus preventing any further groundleakage current. Thus personnel coming into contact with either DCconductor are prevented from receiving a shock of greater than 6 mA AC.Since the DC voltage for a single phase converter is similar to the ACvoltage, this also prevents a DC current flowing of this magnitude. Thepurpose of passing both the conduit and its enclosed conductors throughthe ground leak detector toroid is so that capacitively coupled currentfrom the current-carrying conductors to the conduit, array frame, orbattery box do not cause an imbalance current through the toroid whichcould reduce sensitivity or cause spurious tripping. However, there areinstances when such current due to a gross fault should be detected, aswill be described more fully herein.

A standalone solar installation may also be used in conjunction withutility power. A common form of low-cost installation is where the solarpower is routed via a transfer switch to a “subsistence panel,” which isan electrical load center that supplies power to the most importantappliances that need to be kept running in the event of utility failure.For example, the subsistence panel may feed lights, power points forTVs, radios, computers, fridge, freezer, water pump, and microwave, butmay not attempt to feed heavy consuming appliances such as HVAC, tumbledryer, or electric stove. The transfer switch allows either utility orsolar power to be selected to feed the subsistence panel. Other, heavierloads such as HVAC are connected to a regular load center fed only byutility power. Alternatively, a transfer panel may be used, whichcomprises a number of switches that select, for each appliance orcircuit, whether they take their power from the utility or the solarpanels. The transfer switches can be left in a position wherein thetotal normal load may be handled continuously by the solar array/batterycombination with typically expected hours of sunlight, while preservingthe ability to temporarily feed any other appliances from the solarsystem should an emergency need arise, or feed any load from utility inthe event of a prolonged period without sun.

Existing transfer panels tend to be of one of two types: either theytransfer all loads from one supply to the other, manually orautomatically, which assumes the alternate supply is a generatorpowerful enough to take the whole load, or of a second type that canmanually transfer each circuit independently. The per-circuit type doesnot seem to be readily available with automatic load transfer switching,possibly because no criteria had existed for transferring only someloads and not others. Moreover, the per-circuit type generally comprisestwo sets of circuit breakers, one set for the utility supply and asecond set for the alternate supply. A transfer switch may be connectedafter the breakers, or else the breaker pairs may be mechanicallyinterconnected such that when one of a pair is on, the other is off.Changes to the Electrical Code in 2008 however required the use ofarc-fault breakers (AFCIs) on many circuits, such as bedroom circuits,and many of the remaining circuits use GFCI breakers. AFCI and GFCIcircuit breakers are more expensive, so the aforementioned per-circuittransfer switch load center would now be significantly more costly,since it would require two AFCIs or GFCIs per circuit. Accordingly thereis a need for a more economic form of per-circuit load transferswitching for intelligent load management. The outline of a smarttransfer panel 3000 that provides intelligent load management is shownin FIG. 22.

In FIG. 22, in contrast to the two power busses that normally extenddown the center of the panel and into which circuit breakers areconnected, there are now four power busbars—two for solar power and twofor utility power. Quadruple busbar 3009 is sized to handle 60 amps oneach solar input lug L1 and L2 (3001) and 60 amps on each utility powerinput lug L1 and L2 (3002). The panel of FIG. 22 is intended to be asub-panel and is fed from a 60 A, two-pole breaker in the main servicepanel. The solar input derives from solar inverter 1000 and is alreadycurrent limited. A phase-splitting auto-transformer may be used withinverter 1000 to provide the two anti-phase hot legs L1 and L2 (3001).The transformer need only handle about half the power output of theinverter, if load is evenly balanced between L1 and L2. In a largerinstallation, two load inverters synchronized 180 degrees out-of-phaseand two battery banks can be installed to provide 7.2 Kw on each of L1and L2. One array can be used with combiner 700 to charge both batterybanks alternately, or else two isolated sub-arrays may be used, as it isnot possible to power two anti-phased inverters from the same arraywithout additional isolating devices that cause a few percent additionalefficiency loss. When one array and combiner 700 is used with twoinverters, each inverter may be given first priority control over halfof the strings and second priority control over the other half of thestrings.

Single Pole, Double Throw (SPDT) relays 3003 are used to select powereither from one of the solar power busbars or from a utility powerbusbar. Each Group of 8 relays may be driven by a relay driver chip suchas the aforementioned Texas Instruments part number TPL9201. The circuitbreakers (3004) on one side are wired to select solar power in theunenergized state while the circuit breakers (3005) on the other sideare wired to select utility power in the unenergized state. On eachside, the breakers alternate between selecting L1 and L2, such that apair of adjacent slots may be used for a double pole (240-volt) circuitsuch as a well pump or dryer. The source of power selected by each relay3003 is routed to its associated breaker through a toroidal core 3008upon which a secondary is wound to provide a current sensor for eachcircuit. The current sensor outputs connect to one or more coherentmeasuring chips such as the aforementioned Analog Devices part numberADE7753 which provides a microprocessor interface for reading current,voltage, power, and power factor. The 16 current sensors may bemultiplexed to a single ADE7753 using analog multiplexers and measuredsequentially approximately once every one or two seconds. This issufficiently frequent to capture most circuit on/off switching activityand give a reasonable measure of average power consumption per circuit.The relay coils may be wired to relay driver circuits such as TexasInstrument part number TPL9201. These also have microprocessorinterfaces that permit a microprocessor to control the relays. Amicroprocessor board (not shown) has inputs for the current sensorsignals, power from both solar and utility via small controltransformers, interfaces with any user controls and displays, and withexternal computers such as a PC, and provides control signals to therelays. Software then implements the intelligence to manage the loads,examples of which are as follows.

The user or installer may set up the smart transfer panel uponinstallation either via front panel push buttons or by connecting to aPC which can provide a more user-friendly interface. Set up may includeinitializing a time-of-day clock, determining from which source eachload normally derives its power and setting the criterion for changingthe selection. For example, each circuit can be programmed to be one of:

1. Always solar, never utility.2. Normally solar, priority 1; utility back up.3. Normally solar, priority 2; utility back up.4. Normally utility; priority 1 for solar back up.5. Normally utility, priority 2 for solar back up.6. Always utility, never solar.

The above list is not intended to be exhaustive, and there may be otherregimes, such as different selection depending on time of day, more thantwo priorities, etc. The purpose of priorities is to anticipate what theuser would ultimately want to do in the case of a long utility outageand lack of sun. In the limit when energy runs out, it will be necessaryto shed load in order to prioritize the most important appliances. Forexample, lights could have first priority during the evening andnighttime while well pump, computer and cordless telephone power outletshad first priority for solar energy in daytime. These time-of-daydependent algorithms may be built in with the help of a real time clockchip or software program so that the operation that the user would wishwill be automatic if the occasion arises.

The smart transfer panel may also communicate with inverter 1000 toreceive information on the availability of solar power, which can dependon the state of the battery, which is monitored by the load inverter. Ifutility power is available, i.e., there is no outage requiring emergencypower from solar for loads that would normally be powered from theutility, then it is desirable that the battery would not normally bedischarged to a depth of more than 20% in any day. Therefore, if thebattery reaches a threshold depth of discharge that should not beexceeded except in an emergency, the inverter can inform the smarttransfer panel to begin shedding load from solar and transferring loadto the utility. On the other hand, if utility power fails, those loadswould instantly be transferred back to solar and the battery would bepermitted to be fully discharged. Before full discharge is reachedhowever, at some intermediate point load would start to be shedaccording to the pre-programmed order of priorities.

One feature of the software can be that the smart transfer panelattempts to avoid relay switching under load, in order to maximizecontact life. If transferring power from one source to another is not aninstantaneous imperative, the current sensors can provide an indicationof a low-load period in which to perform load transfer. In addition, theinverter 1000 may be provided with a 60 Hz synchronizing pulse from thetransfer panel so that power transfer is phase-synchronous andsubstantially glitch-free.

The smart transfer panel preferably also has an interface for a PC, forexample, an RS232 interface. A wireless interface using Bluetooth orWiFi is also conceivable. The PC interface circumvents the limit on theamount of program memory that it is economic or sensible to attach to asmall microprocessor such as is envisaged to be included in the smarttransfer panel. Coupling to an external PC allows the PC to hostadditional software and provide full keyboard, mouse and display userinterfaces, color graphics and suchlike. It can also, via the internet,allow remote monitoring or maintenance of an installation, and hasaccess to the entire internet as a resource. For example, weatherforecasts received from the internet can help anticipate solarirradiation (insolation) and manage the energy consumption accordingly.Historical records of insolation by season or date are also available onthe internet for such purposes. The PC interface can also display to theuser his energy consumption on each circuit and how much derived fromsolar and utility respectively, as well as these usages versus time ofday, day of week, month, season, or year. Thus the use of a smarttransfer panel in a hybrid solar/utility power installation can providemany benefits as well as the tools to manage energy consumption forgreater economy.

The sizing of a standalone system is more critical than the sizing of agrid-interactive system, as sufficient power must be provided for themaximum load in the season of minimum insolation, but then the excess iswasted at other times of lower load or higher insolation. A hybridsolar/utility installation with a smart transfer panel and load centeras just described relaxes the need to avoid undersizing the solar sourceand helps to use excess power, but does not derive great benefit fromoversizing. To derive benefit from much larger solar arrays, it caneither be arranged to power some of the heavier house loads such as HVACto beneficially absorb excess solar power, or else a grid-tie invertercan be added to return excess power to the grid. Firstly, a systemhaving only grid-inter-tie will be described with the aid of FIG. 17.

The grid-tie installation shown in FIG. 17 has been deliberately drawnto emphasize the similarity with the standalone installation of FIG. 18.The most noticeable difference is the absence of the battery 500 of FIG.17, and the use of grid-interactive inverter 2200 (single phase version,as illustrated in FIG. 15) as opposed to the standalone inverter 1000.The array now connects directly to the grid-interactive inverter 2200,which, since it is the only user of the array power, connects tocombiner priority 1 output and priority 1 control port. The connectionto the control port is used to provide a remotely-activated DCdisconnect function.

The single-phase grid-interactive inverter impresses substantially thesame common-mode signal of the same 60 Hz frequency on to the DCconductors as the standalone inverter, and thus the same ground leakdetector (800, 801, 802) can be used. It is alternatively possible touse a 60 Hz AC, two-pole GFI breaker on the AC output, the two breaker“line” poles being used unconventionally for the hot and neutral whilethe pigtail and breaker neutral are used to convey the equipment groundto the solar system. This arrangement requires that the breaker beinstalled in a special sub-panel that is wired in the above way, and notin the main service panel (4030). Alternatively the breaker may bepre-wired into converter 2200.

The ground leak detector comprised of items 800, 801, 802 measures theimbalance current between the DC positive line, the DC negative line andthe ground line including the encircling conduit. To do this, theconduit and its entire set of contained conductors passes through thecenter of the toroid. Currents caused on the ground conductor or in theconduit by capacitive coupling of the AC ground-leak probe signals fromthe DC conductors are thereby balanced out and do not cause falsetripping or adversely affect the ground-leak detection sensitivity. Highsensitivity may thereby be achieved for the purposes of protectingpersonnel from shock. For this function, the array end of the conduitand the array frame must not be connected to any local ground, butshould only be grounded via the metallic conduit and the groundingconductor contained therein back through the leak detector to the mainsystem ground.

Current solar systems ignore personnel shock risks and employ groundleak detectors on the DC side purely to warn of malfunction and toprotect against excessive currents. For example, in some systems, theconduit and grounding conductor contained therein may be connected by afuse to the main system ground, which blows in the event of a short tothe conduit or other grounded metal part, such as the array frame,leaving the array and conduit ungrounded. The inverter must switch offunder such circumstances and both DC conductors must be disconnected toisolate the DC circuit and prevent current flow. A warning sign must beplaced at appropriate locations warning that, in the event of a groundfault being indicated, conductors that are normally grounded may nolonger be grounded and may be at hazardous potentials.

Using the inventive ground leak detection principle described herein, animproved form of protection of the DC circuit from excessive currentsmay be contrived. By using a second toroidal core, the DC conductors maybe passed through the core without passing the conduit and groundingconductor through the core. Upon occurrence of a ground fault, the ACprobe signal current in the ground conductor passing around the outsideof the core will not cancel the AC probe current in the DC conductors,but will enhance the coupling to the toroid secondary, thus providing astrong, easily detectable signal in the secondary indicative of a grossground fault that is easily distinguished from capacitive currents byits much larger magnitude. This indication may be detected in theinverter and cause it to immediately shut down, opening the input relaysin the DC circuit, the output relays in the AC output circuit andremoving the relay drive signals to the remote-controlled combiner andDC disconnect. Both variations of the ground leak detector areillustrated in insets A and B of FIG. 17. Inset A shows the conduit aswell as the DC+ and − conductors passing through the toroid, while insetB show only the DC conductors passing through the toroid. The former (A)can detect very small leakage currents to ground other than the conduitor equipment ground on the array side of the detector, while the lattercan detects heavier ground leaks from the DC conductors to the conduitor equipment ground.

When ground leak detectors as just described are used, the AC output ofinverter 2200 is wired to a single-pole, 60 A breaker (4040) in the mainservice panel (4030) or in a conventional sub-panel which connects tothe main service panel. The main service panel (4030) houses the main ACdisconnect for the solar system. When the AC disconnect is operated, theanti-islanding protection required by specification UL1741 operates andshuts down inverter (2200); this in turn removes control power fromcontrol port 1 of combiner (700) causing the DC disconnect relays toopen. An OFF switch on the inverter can also be used to operate the DCdisconnect function, as can a switch locally at combiner 700.

In the grid-tie system of FIG. 17, array 1500 may need to output ahigher DC voltage than in the standalone system of FIG. 18. If thestandalone system of FIG. 18 uses a nominally 120-volt battery, floatcharged at say 135 volts, then the array should deliver powerefficiently at 135 volts. It may also be required to deliver a voltagesomewhere in the range 140 to 144 volts for bulk, absorption andequalization charging. A fuller discussion of array-to-system voltagematching is given later in connection with implementing bimodal systems.

The main service panel of a residential system receives its power from,in the USA, a split-phase 120/240-volt single-phase service drop orservice lateral (4010). The service drop usually comprises threewires—two 120 volt rms hots 180 degrees out-of-phase with each other,and a neutral. The neutral is likely already grounded at the utilitypole transformer or pad-mounted transformer, but is also bonded in themain service panel to the local grounding system provided by groundingelectrode 1010. The two hot lines from the service drop pass through themeter to the main service panel.

One of two possible metering regimes may be used.

In “net metering,” other building loads are supplied by other breakercircuits from main service panel (4030), and the meter clocks up the netdifference between power imported to power the loads and power exportedfrom grid-tie inverter (2200).

In “dual metering,” a separate export meter is connected betweengrid-tie inverter (2200) and the service drop. Thus the service drop(4010) may feed two main service panels through two meters—one for powerimported and one for power exported. Power exported may be remuneratedat a lower or higher rate per kilowatt hour than power imported ischarged, as distinct from net metering, which provides a one-to-oneoffset of imported power by exported power.

FIG. 17 shows a single-phase system typical of a residentialinstallation. However, using 3-phase versions of the inverter that havebeen explained above using FIGS. 16, 23, 24, and 26, a connection to athree phase service drop can be made. Typically in the USA, the 3-phaseservice drop would be a 120/208 volt service, but with suitable voltagescaling, an array and inverter can be adapted to a European 240 volt,single phase service, a European 240/416 3-phase service, a US277/480-volt service, or even a US 120/240-208 volt split-phase plus3-phase service having a 208-volt “high leg.” It is preferred to connecta grid-tie system to the lowest available service voltage in order tominimize the maximum DC voltage of the array for reasons of personnelsafety. When a 120/208 3-phase connection is used, the array voltagesare nominally 294 volts between the positive and negative DC lines and147 volts from each to ground. The shock potential from a 147-volt toground DC line is much less than that of a 120-volt AC line.

Because the grid-tie system of FIG. 17 is the simplest and lowest costto provide, it is the fastest growing type of solar electricinstallation. However, many people are surprised and disappointed tolearn that, if the utility fails, they do not have power. This is partlybecause of the anti-islanding requirement, but also due to the absenceof the battery, which is required to stabilize the array voltage whenthe load is variable. Consequently it is of interest to be able toconstruct or upgrade either FIG. 17 or FIG. 18 to a “bimodal system”which provides both grid tie and battery back-up.

There are many ways to construct a bimodal system. A bimodal invertercannot necessarily operate in both modes simultaneously, although theXANTREX SW4012, 4024 and 4048 can be integrated very well to provideboth modes of operation and automatically switch between the twoappropriately. One way to use the inventive inverters of thisapplication to construct a bimodal system would be to install twoseparate systems according to FIGS. 17 and 18, respectively. However, itis desirable to share parts such as the array 1500, the combiner 700,the DC wiring, (400), the ground leak detector(s) (800, 801, 802), andso forth. FIG. 19 shows a bimodal system sharing these common parts.Since the single-phase grid-tie inverter employs an array voltage of 170volts and the standalone system requires 135-144 volts for batterycharging, a discussion of array voltages, efficiencies, and how bothrequirements can be satisfied is appropriate. Of course, one solution isto operate the standalone inverter at 170 volts, as was shown above tobe perfectly feasible to include during its design, but then a 168 voltbattery comprising 14, 12-volt units in series would be required,instead of the ten mentioned previously to achieve 120 volts. This wouldhave the advantage of providing 16.8 kilowatt hours of back-up capacity,but at greater cost. There is another argument however that can be usedto reconcile different DC voltage requirements of the standalone andgrid-tie inverters.

First of all, the voltage-current characteristics of solar cells will beexplained with the aid of FIGS. 20 and 20 a.

FIG. 20 shows that a photovoltaic cell is a current source ofphotocurrent I1 in parallel with an intrinsic diode D1. In the absenceof an external load, the photocurrent flows through the diode in theforward-biased direction causing the diode to develop a potential acrossits terminals which is equal to the typical voltage drop of a forwardbiased diode of around 0.6 volts. Whatever the technology, diode I-Vcurves are given by Shockley's equation:

I(V,T)=Isat[exp(βqV/kT)−1]  (1)

where I(V,T) is the current at voltage V and temperature T;Isat is the diode saturation voltage (leakage current when reversebiased);q is the charge on an electron;k is Boltzman's constant;T is absolute temperature in degrees Kelvin, andβ is an ideality factor which is between 55% and 80%, i.e., 0.55 to 0.88for silicon diodes.

At 25 degrees C., kT/q is about 26 millivolts, so the equation (1)simplifies to:

I(V,T)=Isat[exp(βV/0.026)−1]  (2)

If the solar cell of FIG. 20 is shorted, the diode voltage will be zeroand will take no current, so all of the photocurrent I1 comes out. Thisis by definition the short circuit current Isc of the cell. When thesolar cell is now open circuited, the current Isc flows into the diodewhich is therefore the value of I(V) given by equation 2, and which canbe inverted to give V, now the open circuit voltage of the solar cellVoc, as:

Voc=(0.026/β)Log_(e)(Isc/Isat+1)  (3)

Since Isc and Voc are externally measurable, the values measured at 25degrees C. in full sun, known as Standard Test Conditions, may besubstituted in equation (3) to obtain the unknown Isat for the diode as:

Isat=Isc/[exp(βVoc/0.026)−1]  (4)

Now substituting for Isat in equation 2, the diode current at any othervoltage V is found to be:

I(V)=Isc[exp(βV/0.026)−1]/[[exp(βVoc/0.026)−1]  (5)

The current output by the solar cell is the photocurrent Isc minus thediode current, that is:

$\begin{matrix}{{{{{\quad{{lout} = {lsc}}\quad}\left\lbrack {1 - {\left\{ {{\exp \left( {{\beta V}/ 0.026} \right)} - 1} \right\}/ \left\{ {{\exp\left( {{\beta {Voc}}/ 0.026} \right)} - 1} \right\}}} \right\rbrack} =}\quad}{\quad{\left\lbrack {1 - {\exp  \left\{ {{\beta \left( {V - {Voc}} \right)}/ 0.026} \right\}}} \right\rbrack / \left\lbrack {1 - {\exp  \left\{ {- {{\beta {Voc}}/ 0.026}} \right\}}} \right\rbrack}}} & (6)\end{matrix}$

A typical monocrystalline silicon solar panel in current productioncomprising 72 series connected cells has the following characteristics:

Maximum Power 195 wattsopen circuit voltage Voc=45 voltsshort circuit current Isc=5.45 ampsvoltage at maximum power Vmp=38.3 voltsCurrent at maximum power Imp=5.09 amps

From the above, the per-cell voltages may be deduced as:

Voc=45/72=0.625 and Vmp=0.532

Substituting in equation 6, the whole I-V curve of this cell may becomputed and the voltage scale multiplied by 72 to give the whole I-Vcurve for the panel. The power may also be plotted by computing theproduct of I and V. This is shown in the curves of FIG. 20a . It isfound that these highly efficient cells have an ideality factor of about74% and a light-to-electricity conversion efficiency over 18%.

The diode voltage of a photovoltaic cell exhibits the same temperaturecoefficient of −2.16 mV per degree C. as other silicon diodes. Solarpanels are deliberately exposed to full sun and therefore tend to riseabove the Standard Test Conditions temperature of 25 degrees C. whenfully illuminated in the summer. Typically, they might reach 50 degreesC., so the open circuit voltage VOC will drop by 25×2.16 mV per cell or3.9 volts for a 72-cell panel. At the coldest winter temperatures, say−15 degrees C., the Voc of the panel may rise by 6.2 volts. These twoextremes are plotted in FIG. 20b . If four of these panels are strung inseries, it can be seen that, at the high temperature extreme, thevoltage range 132-144 volts, which is the exact range desired forbattery charging, is around the peak power point, and peak power isabout 175 watts. If the array is loaded to the same voltage range at thelow temperature extreme, the power is in the range 180-197 watts perpanel. For the single-phase grid interactive inverter requirement of 170volts however, four series panels are not enough at the highesttemperature, although they would be perfect at the low temperatureextreme. If on the other hand 5 panels are strung in series, the 170volts required for the grid-tie inverter lies at the maximum power pointof about 175 watts per panel at the high temperature extreme, while thebattery float charge voltage of 135 volts is sub-optimum, at about 145watts per panel.

In such a bimodal system, the economic benefit is primarily obtained bythe grid-tie system using net metering or dual metering. The purpose ofthe battery and standalone inverter is purely to provide back-up in theevent of a utility outage. Therefore little energy is normally drawnfrom the battery system and the array is only used to keep the batteryfloat charged, for which it is sufficient that the combiner 700occasionally divert one panel from grid-tie to battery chargemaintenance. Therefore the efficiency of battery charging from the arrayis unimportant, and the array voltage should be optimized for bestgrid-tie operation. Five of the illustrative panels would therefore beused in series per string, to provide a maximum power point at 170 voltsat maximum cell temperature, and upon diverting a string for thepurposes of battery float charging, the diverted string would be loadeddown to the slightly less optimum voltage range of 120-140 volts.

The bimodal system of FIG. 19 thus comprises a nominally 170 volt array1500 that operates at its most efficient point when driving the grid-tieinverter. Each string of the array is selectively combined by combiner700 either to feed the grid-tie inverter or to charge the battery. Thebattery is connected to the priority 1 output port of combiner 700 andstandalone converter (1000) controls the string selection for batterycharging via control port 1. Strings not selected for battery chargingare available to be selected by the grid-tie inverter 2200, which wouldusually select all available strings. Thus there are two combinedoutputs from combiner 700 each comprising a DC positive and a DCnegative conductor. These four, high current conductors are containedwithin the same conduit in order to pass together through the same DCleak detector (800, 801, 802). The DC leak detector may be provided witha second winding to feed detectors in both the grid tie inverter and thestandalone inverter, as both will have such detectors built in. Thisprovides continuous ground leak detection, should one or other inverterbe taken out of service or switched off. Standalone converter 1000provides a single hot leg of 7.2 Kw which is split into two anti-phasehot legs by autotransformer 1700, to permit the driving of 240-voltloads as well as 120-volt loads. These two solar-derived hot legsconnect to busses 3001 of Smart Load Center 3000. DC power for grid tieinverter 2200 is split off after the ground leak detector. The AC outputof grid-tie inverter 2200 connects to a 60 A single-pole breaker in theservice panel, or a suitable sub-panel fed from the main service panel.The service panel (or suitable sub-panel) also houses a two-pole breakersupplying utility power to Smart Load Center 3000.

FIG. 19 shows a communication and synchronization buss connecting theSmart Load center 3000, grid-tie inverter 2200 and standalone inverter1000. The communications buss 1750 employs opto-isolators at thereceiving ends to avoid creating ground loops, and serves the followingfunctions:

First, a grid derived 60 Hz sync pulse is supplied to standaloneinverter 1000, of such phase that both grid-tie inverter 2200 andstandalone inverter 1000 are connecting their respective negative DCinput terminals to neutral at the same time, thereby ensuring that thephase of the common mode ground-leak probe signal is the same for both.This is not an essential requirement for operation, but considereddesirable. Moreover, it is desirable to synchronize inverter 1000 withthe utility grid to facilitate glitch-free changeover from utility tosolar by Smart Load Center 3000.

Second, the inverter 1000 can communicate the availability ofsolar/battery power to Smart Load Center 3000 in order to implementprioritized load-shedding in the event of a prolonged utility outage.

Third, the Smart Load Center 3000 can receive status flags from bothinverters 1000 and 2200 and periodic reports of current delivered to thegrid from inverter 2200, so that, in addition to battery status frominverter 1000 and AC circuit current measurements from its ownper-circuit current sensors, all information may be collated forcommunication if desired to a PC via an RS232 connection, and to receivecommands from a PC for remote control of the system via moreuser-friendly software. It is not envisaged that high data rates wouldbe needed on buss 1750, and a baud rate of 60 bauds is envisaged betweenunits 1000, 3000, and 2200 to be sufficient.

It has been shown above how a variety of useful solar energy systems maybe constructed using different combinations of a small number of novelequipment designs. While other industries are competing to reduce thecost of solar panels, soon to be under $1/watt, the inventive invertersand system integration techniques described herein permit commensuratereductions of other main components and balance-of-system components inorder to drive down the total installation cost of a solar system tolevels where the economics of solar energy become compelling.

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive, and allchanges coming within the meaning and equivalency range of the appendedclaims are intended to be embraced therein.

What is claimed is:
 1. A DC to AC converter operative to convert DCpower to a AC power having a desired voltage and waveform, and to outputthe AC power between hot and neutral output terminals, the DC to ACconverter comprising: a plurality of controlled switches, each having apower input connection operative to accept DC power from an associatedDC power source at an associated DC voltage, and each controlled switchfurther having a power output connection operative to output a selectedone of: (a) the associated DC voltage, (b) the associated DC voltagehaving an inverted polarity, and (c) zero voltage, in response to anassociated ternary-valued selection signal representing the multipliervalues +1, −1, or 0 respectively, the power output connections of theplurality of switches being connected in series to output a sum voltageapproximating the desired AC output voltage and waveform.
 2. The DC toAC converter of claim 1 wherein at least some of the DC voltagesassociated with the plurality of controlled switches have differentvalues.
 2. The DC to AC converter of claim 1 wherein the desired voltageis a voltage of a standard household electricity supply and the desiredwaveform is sinusoidal at a standard household electricity supplyfrequency.
 3. The DC to AC converter of claim 1 wherein the controlledswitches are MOSFETs connected in H-bridge configurations, and theassociated DC power sources are relatively floating.
 4. The DC to ACconverter of claim 1 wherein the associated DC voltages differ nominallyby a factor of
 3. 5. The DC to AC converter of claim 1 wherein theternary-valued selection signals comprise pairs of binary bits, each bitpair having in total four combinations of possible values, of which twoof the four combinations represent the zero multiplier value.
 6. The DCto AC converter of claim 1 further comprising: a switch selection signalgenerator operative to produce the ternary-valued selection signals, theswitch selection signal generator being configured to produce sets ofswitch selection signals, at given time instants, such that the sumvoltage output is momentarily the best approximation to theinstantaneous voltage values of the desired waveform at the giveninstants.
 7. The DC to AC converter of claim 1 further comprising: aswitch selection signal generator operative to produce theternary-valued selection signals, the switch selection signal generatorbeing configured to produce sequential sets of switch selection signals,and being configured to produce each new set of switch selection signalsat a time instant at which the new set of switch selection signals wouldcause the sum voltage to be a better approximation to an instantaneousvoltage value of the desired waveform at that time instant than theimmediately previously output set of switch selection signals.
 8. The DCto AC converter of claim 7 further comprising, where the desiredwaveform is repetitive: memory operative to store precomputed sequentialsets of switch selection signals and the associated time instants atwhich each set is to be output; and wherein the switch selection signalgenerator is operative to retrieve the precomputed sequential sets ofswitch selection signals and associated time instants from the memory,and to output the switch selection signals at the associated times. 9.The DC to AC converter of claim 1 wherein the DC power source having thehighest voltage value is a battery and the DC power sources having lowervoltages values comprise bi-directional DC-DC conversion circuitryoperative to derive the lower voltage values from the battery.
 10. TheDC to AC converter of claim 1 wherein the DC power source supplying thehighest mean power is a battery and the DC power sources supplying lowermean power comprise bi-directional DC-DC conversion circuitry operativeto derive the lower mean power from the battery.
 11. The DC to ACconverter of claim 1 wherein one of the power output connections of thecontrolled switch having the associated DC power source of the highestassociated DC voltage is one of the end terminals of the seriesconnection, and is connected to the neutral output terminal.
 12. The DCto AC converter of claim 1 wherein the associated DC power source of oneof the controlled switches is a battery, and further comprising acommon-mode filter interposed between the battery and the controlledswitch.
 13. The DC to AC converter of claim 12 wherein the common modefilter comprises: a capacitor connected between DC power input terminalsof the controlled switch; a common-mode choke connected between thebattery and the DC power input terminals of the controlled switch; afirst pair capacitors connected respectively from positive and negativeterminals of the battery to the neutral output terminal; a dampingresistor connected to the neutral output terminal; and a second pair ofcapacitors connected respectively from the positive and negativeterminals of the battery to the other end of the damping resistor thanthe neutral terminal.
 14. The DC to AC converter of claim 13 furthercomprising an inductor connected in parallel with said damping resistor.15. The DC to AC converter of claim 1 further comprising a low-passoutput filter comprising: an inductor connected between the sum voltageat the end of the series connection of switches and the hot outputterminal; a first capacitor connected across the hot and neutral outputterminals; a damping resistor connected to the neutral output terminal;and a second capacitor connected from the hot output terminal to theother end of the damping resistor than the neutral terminal.
 16. The DCto AC converter of claim 1 further comprising a start-up in-rush currentlimiting circuit interposed between at least one of the controlledswitches and its associated DC power source, the start-up in-rushcurrent limiting circuit comprising: one or more switches operative toinsert a series impedance between the DC power source and the controlledswitch when the DC to AC converter is initially powered on and noelectrical load is connected to an AC output, and further operative toremove the series impedance and connect the DC power source to thecontrolled switch when the in-rush current has dropped below athreshold.